Systems and methods for employing multiple filters to detect t-wave oversensing and to improve tachyarrhythmia detection within an implantable medical device

ABSTRACT

Techniques are described for detecting tachyarrhythmia and also for preventing T-wave oversensing using a narrowband bradycardia filter in combination with a narrowband tachycardia filter. In some embodiments, a separate wideband filter is also exploited. In one illustrative example, ventricular tachycardia (VT) is detected by: detecting a preliminary indication of VT using signals filtered by the bradycardia filter and, in response, confirming the detection of VT using signals filtered by the tachycardia filter. That is, the bradycardia filter, traditionally used only to detect bradycardia, is additionally used to provide a preliminary indication of VT. The tachycardia filter is then activated to confirm the detection of VT before therapy is delivered. In this manner, the tachycardia filter need not run continuously, but is instead activated only when there is some indication of possible VT, and hence power is saved. Numerous other exemplary techniques are set forth herein for arrhythmia detection and for T-wave oversensing detection.

RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. ______,filed Jul. 11, 2007 (Atty. Docket No. A07P1116) entitled “SYSTEMS ANDMETHODS FOR EMPLOYING MULTIPLE FILTERS TO DETECT T-WAVE OVERSENSING ANDTO IMPROVE TACHYARRHYTHMIA DETECTION WITHIN AN IMPLANTABLE MEDICALDEVICE” and is fully incorporated by reference herein.

FIELD OF THE INVENTION

The invention generally relates to implantable medical devices such aspacemakers and implantable cardioverter/defibrillators (ICDs) and, inparticular, to (1) techniques for detecting ventricular tachyarrhythmiaand also to (2) techniques for preventing T-wave oversensing.

BACKGROUND OF THE INVENTION

An arrhythmia is an abnormal heart beat pattern. One example ofarrhythmia is bradycardia wherein the heart beats at an abnormally slowrate or wherein significant pauses occur between consecutive beats.Other examples of arrhythmia include tachyarrhythmias wherein the heartbeats at an abnormally fast rate. With an atrial tachyarrhythmia, suchas atrial tachycardia (AT), the atria of the heart beat abnormally fast.With a ventricular tachyarrhythmia, such as ventricular tachycardia(VT), the ventricles of the heart beat abnormally fast. Though oftenunpleasant for the patient, a tachycardia is typically not fatal.However, some tachycardias, particularly ventricular tachycardia, cantrigger ventricular fibrillation (VF) wherein the heart beatschaotically such that there is little or no net flow of blood from theheart to the brain and other organs. VF, if not terminated, is fatal.Hence, it is highly desirable for implantable medical devices, such aspacemaker or ICDs (herein generally referred to as a pacer/ICD) todetect arrhythmias, particularly ventricular tachyarrhythmias, so thatappropriate therapy can be automatically delivered by the device.

To detect arrhythmias, the pacer/ICD senses electrical cardiac signalswithin the heart of the patient using one or more implanted electrodes.The cardiac signals are sensed within the device by one or more senseamplifiers and then filtered by various filters configured so as toextract signals of interest, such as signals indicative of bradycardiaor tachycardia or other arrhythmias. To this end, state-of-the-artpacer/ICD's are often provided with a wideband filter and two narrowbandwidth filters. The wideband filter eliminates low and high frequencynoise but otherwise retains all features of the cardiac signalsindicative of actual electrical events within the heart of the patient.That is, the wideband filter retains P-waves, R-waves and T-waves,whether occurring at normal heart rates, excessively low rates, orexcessively high rates. The P-wave is the portion of an electricalcardiac signal that is representative of the electrical depolarizationof the atria and is thus also representative of the physical contractionof the atria. The R-wave—which is a part of a QRS complex—is the portionof an electrical cardiac signal that is representative of the electricaldepolarization of the ventricles and is thus also representative of thephysical contraction of the ventricles. The T-wave is the portion of anelectrical cardiac signal that is representative of the electricalrepolarization of the ventricles. Note that the repolarization of theatria typically generates electrical signals that are too weak to bedetected and hence atrial repolarization events are not typicallydetected. Hence, within the wideband cardiac signals, the P-wave istypically followed by the R-wave, which is then followed by the T-wave.Note, however, that the wideband filter also retains signals associatedwith any chaotic or random beating of the chambers of the heart,particularly signals associated with VF, which may not be easilycategorized as having discrete P-waves, R-waves or T-waves. Also, notethat, P-waves, R-waves and T-waves are also features of a surfaceelectrocardiogram (EKG), though the corresponding features of the EKGoften differ in shape and magnitude from those of the IEGM.

FIG. 1 provides a stylized illustration of a cardiac signal 2corresponding to a single heartbeat, particularly illustrating theP-wave 4, R-wave 6, and the T-wave 8. In practice, the relativemagnitudes of the various events can differ significantly. In somecases, the T-wave may be as large as or larger than the R-wave.Accordingly, it can be difficult to obtain an accurate measure of theventricular rate from the wideband-filtered signals and so it can bedifficult to reliably detect either bradycardia or tachycardia from thewideband-filtered signals. Hence, the specialized narrowband filtershave been developed. Initially, a narrowband “bradycardia filter” wasprovided within pacemakers that passed (or “retained”) only R-waves forthe purposes of detecting bradycardia. If the rate at which R-wavesappear in the filtered signal is below a lower rate threshold, or if noR-waves are present at all in the filtered signal, then the patient islikely suffering an episode of bradycardia, and appropriate therapy isdelivered, such as demand-based pacing. Although effective for detectingbradycardia, the filter also eliminates R-waves associated with VF, i.e.the bradycardia filter also filtered out V-fib waves. ICDs need toreliably detect VF for the purposes of delivering defibrillation shocks.Hence, a narrowband “tachycardia” filter was also developed that had awider passband for the purposes of also detecting R-waves or V-fib wavesassociated with VF/VT. If the rate at which R-waves appear in thetachycardia-filtered signal is above a VT threshold, then the patient islikely suffering an episode of VT, and appropriate therapy can bedelivered, such as antitachycardia pacing (ATP). If the rate exceeds ahigher VF threshold, or if V-fib waves are detected, then the patient islikely suffering an episode of VF, and defibrillation shocks aredelivered.

Accordingly, many state-of-the art pacer/ICDs now include both abradycardia filter and a tachycardia filter. Advantageously, becauseT-waves are filtered out by the bradycardia filter, the sensitivity ofthe bradycardia filter can be set quite high so as to permit detectionof even very low amplitude R-waves. The high sensitivity of thebradycardia filter thus substantially eliminates the risk of anypossible undersensing of the R-waves (or at least any significantundersensing of relatively low rate R-waves.) Herein, “undersensing”refers to the failure to detect events of interest that are actuallypresent within the raw cardiac signals. Meanwhile, the elimination ofT-waves means that there is substantially no risk of “oversensing” whenusing the bradycardia filter. Herein, “oversensing” refers to theerroneous detection of an event not actually present in the raw cardiacsignal, such as the detection of R-waves that are not in fact present.Oversensing typically arises when one event is misidentified as another,as may occur, e.g., if a T-wave is improperly identified as an R-wave.As can be appreciated, T-wave oversensing is a significant concern sincemisidentification of T-waves as R-waves can result in significantmiscalculation of the true heart rate within the patient, causingtherapy to be delivered when not warranted or potentially causingtherapy to be withheld even when needed. Insofar as bradycardia isconcerned, T-wave oversensing might result in a failure to detectbradycardia since misidentification of T-waves as R-waves would resultin a significantly higher heart rate being detected than actuallyoccurring within the patient. As noted, the bradycardia filter isconfigured to substantially eliminate all T-waves so that T-waveoversensing is not a concern on the bradycardia channel. Hence, thestate-of-the art pacer/ICD can reliably use the bradycardia filter todetect bradycardia.

FIG. 2 illustrates the operation of the bradycardia filter during normalsinus rhythm. A first graph 10 of the figure illustrates the output ofthe wideband filter, particularly highlighting that portion of thefiltered signal corresponding to the location of T-waves 12. Note that,in the figure, T-waves corresponding to numerous heartbeats are shownsuperimposed over one another. The vertical axis of the graphillustrates the magnitude, in arbitrary units, relative to aniso-electric baseline. The horizontal axis illustrates the time delay inmilliseconds (ms) from the preceding R-wave. A portion of each precedingR-wave appears within the graph around time: 0 ms. A portion of eachsubsequent R-wave appears within the graph as well, beginning at about500 ms. Between them, T-waves are clearly seen.

Meanwhile, a second graph 14 of FIG. 2 illustrates the output of thebradycardia filter. Again, cardiac signals corresponding to numerousheartbeats are shown superimposed over one another. As can be seen,T-waves are completely eliminated within the bradycardia signals,leaving only those portions corresponding to R-waves (again seen at thebeginning and at the end of the highlighted section of the cardiacsignal.) By completely eliminating the T-wave, the ventricular rate canbe easily and accurately measured based solely on the R-waves (at leastat relatively low heart rates), and hence bradycardia can be reliablydetected from the filtered signals.

However, unlike the bradycardia filter, which fully eliminates T-waves,the tachycardia filter retains T-waves. This is due to the fact that thefrequencies associated with the V-fib waves of interest are alsoassociated with T-waves, and hence the filter cannot eliminate allT-waves while still retaining the V-fib waves. As such, the sensitivityof the tachycardia filter must be set so as to detect high rate R-wavesand V-fib waves while eliminating T-waves. This is difficult, at best,since the relative magnitudes of the R-waves, V-fib waves and T-wavesmay change significantly over time within the patient, perhaps due tothe use of medications or due to physiological or anatomical changes inthe heart brought on by medical conditions, such as cardiac ischemia,myocardial infarctions, congestive heart failure, etc. Moreover, asalready noted, T-waves can sometimes have a magnitude that equals orexceeds that of the R-wave. Hence, T-wave oversensing is a significantproblem within the tachycardia-filtered signals.

FIG. 3 illustrates the operation of a tachycardia filter during VT/VF. Afirst graph 16 illustrates the output of the wideband filter, againparticularly highlighting that portion of the filtered signalcorresponding to the location of T-waves 18. Note that, as in theprevious figure, T-waves corresponding to numerous heartbeats are shownsuperimposed over one another. The vertical axis of the graph againillustrates magnitude relative to an iso-electric baseline. Thehorizontal axis again illustrates the time delay from the precedingR-wave. A portion of each preceding R-wave appears within the grapharound time: 0 ms-10 ms. A portion of each subsequent R-wave appearswithin the graph as well, beginning at about 60 ms. Between them,T-waves 18 are clearly seen. (As a result of variations in R-R intervalsoccurring during VT, the R-waves and T-waves from different heartbeatsare not aligned with one another within the graph, as was the case inFIG. 1.)

Meanwhile, a second graph 20 of FIG.2 illustrates the output of thetachycardia filter. Again, cardiac signals corresponding to numerousheartbeats are shown superimposed over one another. As can be seen,T-waves 22 are not completely eliminated within the tachycardia-filteredsignals, leaving signals that might be misidentified as R-waves,particularly if the sensitivity of the tachycardia filter is set toohigh. That is, T-wave oversensing might occur. Without completeelimination of the T-wave, the ventricular rate cannot be accurately andreliably measured based solely on the output of the tachycardia filter(at least at the rates associated with VT/VF), and hence problems arisein the detection of VT/VF or other forms of ventricular tachyarrhythmia.Failure to properly detect VT/VF when it is present can result in afailure to deliver appropriate therapy. False detection of VT/VF when itis not present can result in delivery of inappropriate therapy. As canbe appreciated, both situations are of significant concern.

In view of the problems arising when using a narrowband tachycardiafilter, it is highly desirable to provide improved techniques forreliably detecting VT/VF that may be performed by a pacer/ICD. It is tothis end that various aspects of the invention are generally directed.It is particularly desirable to provide improved techniques that do notrequire replacement or elimination of existing tachycardia filters, butthat instead achieve improved VT/VF detection when using otherwiseconventional tachycardia filters. It is to this end that particularaspects of the invention are directed.

Still further aspects of the invention are directed to providingimproved techniques for detecting and eliminating T-wave oversensing,even in the absence of any arrhythmia. Heretofore, at least sometechniques for addressing T-wave oversensing have been directed toproviding blanking intervals synchronized with the expected location ofthe T-wave. See, for example, U.S. Pat. No. 6,862,471 to McClure, etal., entitled “Method and Apparatus for Blanking T-Waves from CombipolarAtrial Cardiac Signals based on Expected T-Wave Locations.” It would bedesirable to provide techniques for detecting and eliminating T-waveoversensing that do not necessarily require the use of blankingintervals, and various aspects of the invention are directed to that endas well.

SUMMARY OF THE INVENTION

In a first general embodiment of the invention, a method is provided fordetecting tachyarrhythmia within a patient in which an implantablemedical device is implanted, where the device is equipped to processelectrical cardiac signals sensed via leads implanted within the patientand wherein the device has a first filter operative to substantiallyeliminate signals having frequencies associated with ventricularrepolarization events while retaining signals having frequenciesassociated with at least some ventricular depolarization events and asecond filter operative to pass signals having frequencies associatedwith ventricular depolarization events and ventricular repolarizationevents. In the illustrative embodiments described herein, the firstfilter is referred to as a bradycardia filter and the second filter isreferred to as a tachycardia filter.

The first general embodiment comprises: sensing electrical cardiacsignals within the patient; selectively filtering the signals using thebradycardia filter and the tachycardia filter; and then detectingtachyarrhythmia within the patient using signals filtered by thebradycardia filter in combination with signals filtered by thetachycardia filter. In other words, a tachyarrhythmia, such as VT, isdetected based on a combination of bradycardia-filtered signals andtachycardia-filtered signals. This is in contrast with the predecessortechniques described above, wherein tachycardia is detected using onlythose signals sensed by the tachycardia filter.

In a first illustrative example of the first general embodiment of theinvention, tachyarrhythmia is detected using signals filtered by thebradycardia filter in combination with signals filtered by thetachycardia filter by: detecting a preliminary indication oftachyarrhythmia using signals filtered by the bradycardia filter; and,in response, confirming the detection of tachyarrhythmia using signalsfiltered by the tachycardia filter. That is, the bradycardia filter,which is traditionally used only to detect bradycardia, is additionallyused to detect a preliminary indication of a tachyarrhythmia, such asVT. If such a preliminary indication is detected, the tachycardia filteris then activated to confirm the detection of the tachyarrhythmia,before therapy is delivered. In this manner, the tachycardia filter neednot run continuously, but is instead activated only when there is someindication of possible tachyarrhythmia, and hence power is saved. In onespecific example, a single filter is employed that is capable of beingprogrammed to operate as either a bradycardia filter or a tachycardiafilter. By default, it operates as a bradycardia filter. If anindication of tachyarrhythmia is detected, it is then reprogrammed toinstead operate as a tachycardia filter. In this manner, a singlereconfigurable filter can be used to perform the functions of bothbradycardia filtering and tachycardia filtering, thus saving deviceresources.

In the first illustrative example, the preliminary indication oftachyarrhythmia may be detected by analyzing ventricular channel signalsfiltered by the bradycardia filter to detect one or more of: aventricular rate that exceeds a predetermined VT detection threshold;the presence of a significant number of ventricular depolarizationevents of irregular shape; the presence of a significant number ofventricular depolarization events of irregular size; the presence of asignificant number of ventricular depolarization events occurring at arate below the VT detection threshold but above a rate consistent withnormal sinus rhythm; or the lack of ventricular depolarization events,wherein the lack of ventricular depolarization events is not consistentwith bradycardia (as may occur during VF.) Once a preliminary indicationof tachyarrhythmia has been detected, ventricular channel signalsfiltered by the tachycardia filter are then analyzed to confirm theventricular tachyarrhythmia by, e.g., determining a ventricular rateusing the signals filtered by the tachycardia filter and then verifyingthat the ventricular rate exceeds the VT detection threshold.

In a second illustrative example of the first general embodiment of theinvention, tachyarrhythmia is detected using signals filtered by thebradycardia filter in combination with signals filtered by thetachycardia filter by: filtering ventricular channel signals sensed viathe leads using the tachycardia filter while also filtering ventricularchannel signals sensed via the leads using the bradycardia filter; andthen comparing the ventricular channel signals filtered by thetachycardia filter and the bradycardia filter to detect ventriculartachyarrhythmia. In other words, in this embodiment, the bradycardia andtachycardia filters preferably operate simultaneously to filter the samesignals. The filtered signals are compared to detect thetachyarrhythmia. In one particular example, ventricular tachyarrhythmiais detected by: (1) determining a tachycardia filter-based ventricularrate from the signals filtered by the tachycardia filter while alsodetermining a bradycardia filter-based ventricular rate from the signalsfiltered by the bradycardia filter; (2) comparing the tachycardiafilter-based ventricular rate to a predetermined VT detection threshold;(3) comparing the tachycardia filter-based ventricular rate to thebradycardia filter-based ventricular rate; and (4) detecting aventricular tachyarrhythmia if the tachycardia filter-based ventricularrate is greater than the VT threshold and if the tachycardiafilter-based ventricular rate is also greater than twice the bradycardiafilter-based ventricular rate. If so, VT therapy is immediatelydelivered. If, instead, the tachycardia filter-based ventricular rate isgreater than the VT threshold but not greater than twice the bradycardiafilter-based ventricular rate, then additional confirmation proceduresare employed to verify the VT before therapy is delivered. If,alternatively, the tachycardia filter-based ventricular rate is notgreater than the VT threshold but is about equal to twice thebradycardia filter-based ventricular rate, then an indication of T-waveoversensing is generated.

In this regard, if the ventricular rate derived from the tachycardiafilter is about equal to twice the ventricular rate derived bradycardiafilter, the tachycardia filter rate is likely due to T-wave oversensing,i.e. each T-wave is being misidentified as an R-wave, yielding a ratedouble that of the bradycardia filter. In that case, tachycardia isprobably not actually occurring and so an indication of T-waveoversensing by the tachycardia filter is generated. Nevertheless, if theventricular rate is above the VT threshold then, to be safe, VTconfirmation procedures are preferably initiated to determine whether atachycardia might be occurring and, if so, appropriate therapy isdelivered. However, if the ventricular rate derived from the tachycardiafilter is above the VT threshold and is also greater than twice theventricular rate derived bradycardia filter, then tachycardia is almostcertainly occurring, since T-wave oversensing, by itself, would notproduce such a result. Accordingly, as set forth in step (4) above, ifthe ventricular rate derived from the tachycardia filter is greater thanthe VT threshold and if the ventricular rate derived from thetachycardia filter is also greater than twice the ventricular ratederived from the bradycardia filter, then VT is immediately detected,i.e. no further confirmation is required, and so therapy is promptlydelivered. Also, note that, if the rate derived from the tachycardiafilter is well below the VT threshold and is also about equal to therate derived from the bradycardia filter, then normal sinus rhythm isoccurring without T-wave oversensing and so no action need be taken.

This logic is summarized in Table I. (Note that not all possible logiccombinations of the parameters are set forth in the Table. Rather, onlythose logic combinations that are pertinent to the second illustrativeexample are set forth.)

TABLE I Tachycardia Tachycardia Filtered Rate Filtered Rate versusversus Bradycardia VT Threshold Filtered Rate Result TachycardiaTachycardia Rate > 2 * → VT Immediately Rate > VT Bradycardia RateDetected; Threshold Deliver Therapy Tachycardia Tachycardia Rate ≦ 2 * →Possible T-wave Rate > VT Bradycardia Rate oversensing; ThresholdPossible VT; Initiate VT/VT Confirmation Procedure Before DeliveringTherapy Tachycardia Tachycardia Rate ≈ → T-wave Rate ≦ VT 2 *Bradycardia Oversensing; Threshold Rate Adjust Tachycardia FilterSensitivity Tachycardia Tachycardia Rate ≈ → No T-wave Rate << VTBradycardia Rate Oversensing Threshold Normal Sinus Rhythm

Thus, with this implementation, by simultaneously using both abradycardia and a tachycardia filter and comparing the two filteredsignals to one another, very prompt detection of VT can be achieved,while also detecting possible T-wave oversensing.

In a third illustrative example of the first general embodiment of theinvention, tachyarrhythmia is detected by: filtering ventricular channelsignals sensed via the leads using the tachycardia filter; detecting apreliminary indication of tachyarrhythmia using the signals filtered bythe tachycardia filter; and, in response, confirming the detection oftachyarrhythmia by comparing additional signals filtered by thetachycardia filter with additional signals filtered by the bradycardiafilter. The logic summarized above may be exploited to confirm thedetection of tachyarrhythmia using the signals filtered by thetachycardia filter and the signals filtered by the bradycardia filter.In other words, this embodiment is similar to the second illustrativeexample, but the tachycardia filter is used to detect a preliminaryindication of tachyarrhythmia before any bradycardia-filtered signalsare compared against tachycardia-filtered signals. The preliminaryindication may be used, e.g., to trigger charging of defibrillationcapacitors in the case that a defibrillation shock is ultimatelyrequired. The preliminary indication of tachyarrhythmia may be detectedby determining a ventricular rate based on the signals filtered by thetachycardia filter and comparing that rate against a VT threshold. Inany case, by performing the comparison of the bradycardia filteredsignals and the tachycardia-filtered signals only if a preliminaryindication of tachyarrhythmia has already been made, such a comparisonneed not be performed in the absence of possible tachyarrhythmia.

In a fourth illustrative example of the first general embodiment of theinvention, tachyarrhythmia is detected by: comparing ventricular channelsignals filtered by the bradycardia filter with ventricular channelsignals filtered by the tachycardia filter to distinguish between “true”ventricular events and “false” ventricular events; and then detectingtachyarrhythmia based on the true ventricular depolarization events. Inone example, true ventricular depolarization events are distinguishedfrom false ventricular depolarization events by: filtering ventricularchannel signals using the bradycardia filter and identifying ventricularevents therein; filtering ventricular channel signals using thetachycardia filter and identifying ventricular events therein; detectinga first ventricular event in either of the filtered signals; determiningwhether a second ventricular event occurs within the signals filtered bythe tachycardia filter within a predetermined time window following thefirst event; and if so, identifying the second event as being a falseventricular depolarization event indicative of tachycardia-filteroversensing, and if not, identifying the second event as beingindicative of a true ventricular depolarization event. In other words,following detection of an event within the tachycardia filtered eithersignals or the bradycardia-filtered signals, the device opens up adetection window. If another event is detected within thetachycardia-filtered signals within that detection window, the secondevent is rejected as being a T-wave. Otherwise, the second event isdeemed to be a true R-wave. The time window may be, for example, set inthe range of 50-150 ms.

In one example, tachyarrhythmia is detected based on the trueventricular depolarization events by calculating a ventricular ratebased only on true R-waves and comparing that rate to a VT threshold. Inanother example, tachyarrhythmia is detected based on all ventricularevents by: determining a ventricular rate based on all detectedventricular events (i.e. true and false depolarization events); countinga number of false ventricular depolarization events within apredetermined number of combined false and true depolarization events;generating an indication of tachycardia filter-oversensing if the countexceeds a predetermined count threshold indicative of tachycardiafilter-oversensing; and then detecting ventricular tachyarrhythmia ifthe ventricular rate exceeds a VT threshold and the count does notexceed the predetermined count threshold. For example, if at least sevenfalse R-waves are detected out of every ten total R-waves, then T-waveoversensing is deemed to be occurring and so ventricular tachyarrhythmiais not initially indicated, even if the rate exceeds the VT threshold,due to the significant T-wave oversensing. Preferably, confirmationprocedures are then employed to determine whether VT is neverthelessoccurring, despite the T-wave oversensing.

In another example, the step of comparing ventricular channel signalsfiltered by the bradycardia filter with ventricular channel signalsfiltered by the tachycardia filter to distinguish between trueventricular depolarization events and false ventricular depolarizationevents is performed only in response to detection of a preliminaryindication of tachyarrhythmia made using the tachycardia filter. In yetanother example, the steps of (a) comparing ventricular channel signalsfiltered by the bradycardia filter with ventricular channel signalsfiltered by the tachycardia filter to distinguish between trueventricular depolarization events and false ventricular depolarizationevents and (b) detecting tachyarrhythmia based on the true ventriculardepolarization events are only performed during a “confirmation period”following the preliminary detection of tachyarrhythmia made using thetachycardia filter. The confirmation period may extend, e.g., for 100ventricular event cycles following that preliminary detection. That is,upon detection of a possible VT made using the tachycardia filter, thedevice then seeks to confirm the arrhythmia using both the tachycardiaand bradycardia filters during the next 100 cardiac cycles. If thearrhythmia is confirmed during that period of time, therapy is deliveredpromptly. If the arrhythmia is disconfirmed during that period of time(due to detection of significant T-wave oversensing), therapy is notdelivered. To be safe, if thereby is neither confirmed nor disconfirmedduring that period of time, but the ventricular rate remains above theVT threshold, therapy is promptly delivered at the end of that period oftime. This ensures that therapy is delivered in circumstances where itmay not be clear whether T-wave oversensing is occurring or not.

In a fifth illustrative example of the first general embodiment of theinvention, wherein the device additionally includes a wideband filterhaving a substantially wider bandwidth than bandwidths of thebradycardia and tachycardia filters, tachyarrhythmia is detected usingsignals filtered by the wideband filter in combination with signalsfilters by the bradycardia and tachycardia filters. That is, all threefilters are exploited. In one example, tachyarrhythmia is detected by:identifying possible ventricular depolarization events within signalsfiltered, respectively, by the wideband filter, the bradycardia filter,and the tachycardia filter; comparing the timing of the possibleventricular depolarization events identified within the respectivefiltered signals to identify true ventricular depolarization events; andthen detecting ventricular tachyarrhythmia based on the true ventriculardepolarization events.

In this regard, events that occur substantially contemporaneously withinsignals filtered by the wideband filter, the bradycardia filter, and thetachycardia filter are identified as being true ventriculardepolarization events. Events that occur substantially contemporaneouslywithin signals filtered by the wideband filter and the tachycardiafilter but not the bradycardia filter are identified as being“tachycardia filter-based anomalous events” indicative of (a) a possibleventricular repolarization event (i.e. T-wave) oversensed on thetachycardia filter or (b) a possible ventricular depolarization event(i.e. R-wave) occurring during VF. In response to a tachycardiafilter-based anomalous event, the device determines if a ventricularrate derived from the wideband filter is consistent with VF and, if so,the device delivers VF therapy and, if not, the device rejects theanomalous event for the purposes of ventricular rate calculation asbeing an oversensed ventricular repolarization event (i.e. a T-wave) andthen adjusts the sensitivity of the tachycardia filter to reduceoversensing. Events that occur substantially contemporaneously withinsignals filtered by the wideband filter and the bradycardia filter butnot the tachycardia filter are identified as being “bradycardiafilter-based anomalous events” indicative of possible tachycardia-filterundersensing. In response to tachycardia filter undersensing, the deviceadjusts the sensitivity of the tachycardia filter to reduce suchundersensing. If an event is detected on the wideband filter, but not oneither the tachycardia filter or the bradycardia filter, that event isignored as either noise or a far-field P-wave.

This logic is summarized in Table II. (Although not shown in the table,in the unlikely event that an event is detected on both the bradycardiaand tachycardia filters but not on the wideband filter, that eventignored as an anomalous event, likely arising due to noise on thebradycardia and tachycardia channels.)

TABLE II Wideband Tachycardia Bradycardia Filtered Signal FilteredSignal Filtered Signal Result Event Detected Event Detected EventDetected → True R-wave Event Detected Event Detected Event Not →possible Detected T-wave oversensed with the tachycardia filter possibleR- wave occurring during VF Event Detected Event Not Event Detected →Possible Detected tachycardia filter undersensing Event Detected EventNot Event Not → Noise or far-field Detected Detected P-wave on widebandfilter

Thus, a variety of techniques are provided for detecting ventriculartachyarrhythmias. Various aspects of the invention can potentially beextended to detecting atrial tachyarrhythmias as well. Also, the varioustechniques can be selectively combined to further improve thespecificity with which arrhythmias are detected. The various techniquesmay be implemented, where appropriate, as systems, methods or otherappropriate embodiments.

In a second general embodiment of the invention, a method is providedfor detecting the oversensing of ventricular repolarization events (i.e.T-waves) within a patient in which an implantable medical device isimplanted, where the device is equipped to process electrical cardiacsignals sensed via leads implanted within the patient and wherein thedevice has both a bradycardia filter and a tachycardia filter forfiltering the signals. The second general embodiment comprises: sensingelectrical cardiac signals within the patient; selectively filtering thesignals using a bradycardia filter and a tachycardia filter; anddetecting the oversensing of ventricular repolarization events withinthe signals filtered by the tachycardia filter by comparing the signalsfiltered by the tachycardia filter with the signals filtered by thebradycardia filter. In other words, T-wave oversensing is detected basedon a combination of bradycardia-filtered signals andtachycardia-filtered signals. This is in contrast with the predecessordesigns described above, wherein blanking intervals are employed.

In a first illustrative example of the second general embodiment of theinvention, the signals are selectively filtered using the bradycardiafilter and the tachycardia filter by filtering ventricular channelsignals using the bradycardia filter and determining a bradycardiafilter-based ventricular rate, and also filtering ventricular channelsignals using the tachycardia filter and determining a tachycardiafilter-based ventricular rate. The oversensing of T-waves is detected bycomparing the tachycardia filter-based ventricular rate to thebradycardia filter-based ventricular rate, and then detectingoversensing of ventricular repolarization events within signals filteredby the tachycardia filter by determining if the tachycardia filter-basedventricular rate is about twice the bradycardia filter-based ventricularrate. In this regard, if T-wave oversensing is occurring, each T-wavemay be misidentified as an R-wave, resulting in a tachycardia-filteredventricular rate about twice that of the bradycardia-filteredventricular rate. Accordingly, if the tachycardia filter-basedventricular rate is about twice the bradycardia filter-based ventricularrate T-wave oversensing is almost certainly occurring. See Table IIabove.

In a second illustrative example of the second general embodiment of theinvention, the signals are selectively filtered using the bradycardiafilter and the tachycardia filter by filtering ventricular channelsignals using the bradycardia filter and identifying ventricular eventstherein, and also filtering ventricular channel signals using thetachycardia filter and identifying ventricular events therein. Theoversensing of T-waves is detected by determining, upon detection of afirst ventricular event either in the signals filtered by thebradycardia filter or in the signals filtered by the tachycardia filter,whether a second ventricular event is detected in the signals filteredby the tachycardia filter within a predetermined time window followingthe first event. If so, the second event is identified as being a falseventricular depolarization event indicative of tachycardia-filteroversensing. If not, the second event is identified as being indicativeof a true ventricular depolarization event. In other words, followingdetection of an R-wave within either the tachycardia-filtered signals orthe bradycardia-filtered signals, the device opens up a detectionwindow. If another R-wave is detected within the tachycardia-filteredsignals within that detection window, the second R-wave is rejected asbeing a T-wave. Otherwise, the second R-wave is deemed to be a trueR-wave. The time window may be, for example, set in the range of 50-150ms. Similar techniques are discussed above in connection withtachyarrhythmia detection under the fourth illustrative example of thefirst general embodiment of the invention.

In a third illustrative example of the second general embodiment of theinvention, wherein the device additionally includes a wideband filterhaving a substantially wider bandwidth than bandwidths of thebradycardia and tachycardia filters, the oversensing of T-waves isdetected by identifying possible ventricular depolarization events(R-waves) within signals filtered, respectively, by the wideband filter,the bradycardia filter, and the tachycardia filter; and then bycomparing the timing of the possible ventricular depolarization eventsidentified within the respective filtered signals to identify oversensedventricular repolarization events. In one example, the timing of thepossible ventricular depolarization events is compared to identifyoversensed ventricular repolarization events by identifying events thatoccur substantially contemporaneously within signals filtered by thewideband filter, the bradycardia filter, and the tachycardia filter asbeing true ventricular depolarization events. Such events are deemed tobe ventricular depolarization events (i.e. R-waves) and not oversensedT-waves. In another example, the timing of the possible ventriculardepolarization events is compared to identify oversensed ventricularrepolarization events by: identifying events that occur substantiallycontemporaneously within signals filtered by the wideband filter and thetachycardia filter but not the bradycardia filter as being a tachycardiafilter-based anomalous event indicative of one or more of (a) a possibleventricular repolarization event (T-wave) oversensed on the tachycardiafilter and (b) a possible ventricular depolarization event (R-wave)occurring during ventricular fibrillation (VF). In response to atachycardia filter-based anomalous event, the device determines if aventricular rate derived from the wideband filter is consistent with VFand, if so, delivers VF therapy and, if not, rejects the anomalous eventfrom ventricular rate calculation as being an oversensed ventricularrepolarization event (T-wave). Similar techniques are discussed above inconnection with tachyarrhythmia detection under the fifth illustrativeexample of the first general embodiment of the invention.

Thus, a variety of techniques are provided for detecting T-waveoversensing. The various techniques can be selectively combined tofurther improve the specificity with which T-wave oversensing isdetected. The various techniques may be implemented, where appropriate,as systems, methods or other appropriate embodiments. The T-wavesoversensing detection techniques and the tachyarrhythmia detectiontechniques may be combined, as already set forth in the precedingsummary.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further features, advantages and benefits of the inventionwill be apparent upon consideration of the descriptions herein taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating a cardiac signal, particularlyidentifying P-waves, R-waves, and T-waves therein;

FIG. 2 includes graphs of exemplary filtered cardiac signalsillustrating the operation of a wideband filter and a narrowbandbradycardia filter, and particularly illustrating the complete filteringof T-waves by the bradycardia filter;

FIG. 3 includes graphs of exemplary filtered cardiac signalsillustrating the operation of the wideband filter and a narrowbandtachycardia filter, and particularly illustrating the partial filteringof T-waves by the tachycardia filter;

FIG. 4 illustrates pertinent components of an implantable medical systemhaving a pacer/ICD capable of detecting tachyarrhythmias based onsignals filtered using a narrowband bradycardia filter (FIG. 2) incombination with signals filtered using a narrowband tachycardia filter(FIG. 3) and capable of delivering therapy in response thereto, andfurther capable of detecting T-wave oversensing also based on signalsfiltered using the narrowband bradycardia filter in combination withsignals filtered using a narrowband tachycardia filter;

FIG. 5 provides an overview of a technique performed by the system ofFIG. 4 for detecting tachyarrhythmias using the bradycardia filter incombination with the tachycardia filter and for delivering therapy inresponse thereto;

FIG. 6 illustrates a first illustrative example of the tachyarrhythmiadetection technique of FIG. 5, wherein the bradycardia filter is used toprovide a preliminary indication of tachyarrhythmia, which is thenconfirmed using the tachycardia filter;

FIG. 7 particularly illustrates techniques for detecting a preliminaryindication of tachyarrhythmia using the bradycardia filter for use withthe embodiment of FIG. 6;

FIG. 8 particularly illustrates techniques for confirming the detectionof tachyarrhythmia using the tachycardia filter also for use with theembodiment of FIG. 6;

FIG. 9 illustrates a second illustrative example of the tachyarrhythmiadetection technique of FIG. 5, wherein signals filtered by thebradycardia filter are compared with signals filtered by the tachycardiafilter to detect tachyarrhythmia;

FIG. 10 particularly illustrates techniques for comparing the signalsfiltered by the bradycardia filter with the signals filtered by thetachycardia filter for use with the embodiment of FIG. 9;

FIG. 11 particularly illustrates techniques for addressing possibleT-wave oversensing for use with the techniques of FIG. 10;

FIG. 12 illustrates a third illustrative example of the tachyarrhythmiadetection technique of FIG. 5, wherein a preliminary indication oftachyarrhythmia is made using the tachycardia filter and then additionalsignals filtered by the bradycardia filter are compared with additionalsignals filtered by the tachycardia filter to confirm detection of thetachyarrhythmia;

FIG. 13 illustrates a fourth illustrative example of the tachyarrhythmiadetection technique of FIG. 5, wherein a preliminary indication oftachyarrhythmia is made using the tachycardia filter and then additionalsignals filtered by the bradycardia filter are compared with additionalsignals filtered by the tachycardia filter to identify true R-waves (asopposed to oversensed T-waves) so as to permit detection oftachyarrhythmia based only on true R-waves;

FIG. 14 particularly illustrates techniques for distinguishing true andfalse R-waves for use with the embodiment of FIG. 13;

FIG. 15 particularly illustrates techniques for confirmingtachyarrhythmias based on the true R-waves for use with the embodimentof FIG. 13;

FIG. 16 illustrates a fifth illustrative example of the tachyarrhythmiadetection technique of FIG. 5, wherein signals filtered by a bradycardiafilter, a tachycardia filter and a wideband filter are compared toidentify true R-waves so as to permit detection of tachyarrhythmia basedonly on true R-waves;

FIG. 17 particularly illustrates techniques for comparing the timing ofevents detected using the three filters to distinguish true R-waves fromother events for use with the embodiment of FIG. 16;

FIG. 18 particularly illustrates techniques for distinguishing T-wavesoversensed on the tachycardia filter from R-waves occurring during VFfor use with the techniques of FIG. 17;

FIG. 19 provides an overview of a technique performed by the system ofFIG. 4 for detecting T-wave oversensing using a bradycardia filter incombination with a tachycardia filter;

FIG. 20 illustrates a first illustrative example of the T-waveoversensing technique of FIG. 19, wherein T-wave oversensing is detectedif a tachycardia filter-based ventricular rate is about twice abradycardia filter-based ventricular rate.

FIG. 21 illustrates a second illustrative example of the T-waveoversensing technique of FIG. 19, wherein a detection window is employedto detect T-wave oversensing.

FIG. 22 illustrates a third illustrative example of the T-waveoversensing technique of FIG. 19, wherein signals filtered by abradycardia filter, a tachycardia filter and a wideband filter arecompared to detect T-wave oversensing;

FIG. 23 is a simplified, partly cutaway view, illustrating the pacer/ICDof FIG. 4 along with a set of leads implanted in the heart of a patient;

FIG. 24 is a functional block diagram of the pacer/ICD of FIG. 23,illustrating basic circuit elements that provide cardioversion,defibrillation and/or pacing stimulation in four chambers of the heartand particularly illustrating an components for detectingtachyarrhythmias and detecting T-wave oversensing in accordance with thetechniques of FIGS. 5-22;

FIG. 25 is a functional block diagram of pertinent components of thepacer/ICD of FIG. 24, particularly illustrating components for detectingtachyarrhythmias in accordance with the techniques of FIGS. 5-18; and

FIG. 26 is a functional block diagram of pertinent components of thepacer/ICD of FIG. 24, particularly illustrating components for detectingT-wave oversensing in accordance with the techniques of FIGS. 19-22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode presently contemplatedfor practicing the invention. This description is not to be taken in alimiting sense but is made merely to describe general principles of theinvention. The scope of the invention should be ascertained withreference to the issued claims. In the description of the invention thatfollows, like numerals or reference designators are used to refer tolike parts or elements throughout.

Overview of Implantable Medical System

FIG. 4 illustrates an implantable medical system 24 having a pacer/ICDthat includes a bradycardia/tachycardia filter-based tachyarrhythmiadetection system, i.e., a system capable of detecting tachyarrhythmiasbased on signals filtered using a narrowband bradycardia filter incombination with signals filtered using a narrowband tachycardia filter,and also capable of detecting T-wave oversensing based on signals fromthe narrowband bradycardia and tachycardia filters. To this end,pacer/ICD 26 receives voltage signals from various cardiac pacing leads28 (only two of which are shown in the FIG. 4) from which variouschannels of cardiac signals (herein referred to as intracardiacelectrogram (IEGM) signals) are derived including, for example, unipolaror bipolar atrial IEGM (A-IEGM) signals and unipolar or bipolarventricular IEGM (V-IEGM) signals. A complete set of exemplary pacingleads are shown in FIG. 23 from which a wide variety of specificchannels of IEGM signals may be derived. The signals are selectivelyfiltered using a narrowband bradycardia filter and a narrowbandtachycardia filter to generate filtered signals by which tachyarrhythmiais detected and T-wave oversensing is detected. That is, the bradycardiafilter is exploited (in combination with the tachycardia filter) todetect tachyarrhythmias and also to detect T-wave oversensing occurringon tachycardia filtered channels. As will be explained below, a widebandfilter may be exploited as well, both in the detection oftachyarrhythmias and in the detection of T-wave oversensing.

The pacer/ICD is also capable of delivering therapy in response totachyarrhythmias, such as delivery of antitachycardia pacing (ATP) inresponse to VT or the delivery of high voltage defibrillation shocks inresponse to VF. Diagnostic information pertaining to any detectedtachyarrhythmias and to the detection of any T-wave oversensing may bestored within the pacer/ICD for transmission to a bedside monitor 30, ifone is provided, or for subsequent transmission to an externalprogrammer (not shown in FIG. 4) for review by a physician or othermedical professional. The physician may then prescribe any otherappropriate therapies to prevent additional episodes oftachyarrhythmias. The physician may also adjust the operation of thepacer/ICD to activate, deactivate or otherwise control any therapiesthat are automatically applied and to adjusting the operation of thefilters, if needed, to address any T-wave oversensing problems. Thebedside monitor may be directly networked with a centralized computingsystem, such as the HouseCall™ system of St. Jude Medical, for promptlynotifying the physician of any abnormal conditions, particularly anylife-threatening ventricular tachyarrhythmias. Networking techniques foruse with implantable medical systems are set forth, for example, in U.S.Pat. No. 6,249,705 to Snell, entitled “Distributed Network System forUse with Implantable Medical Devices.”

Thus, FIG. 4 provides an overview of an implantable system for detectingtachyarrhythmias and for delivering therapy in response thereto and fordetecting T-wave oversensing. Although a pacer/ICD is illustrated inFIG. 4, it should be understood that the techniques of the invention maybe implemented within other implantable medical devices.

Overview of Tachyarrhythmia Detection Techniques

FIG. 5 provides an overview of the techniques of the invention fordetecting tachyarrhythmias using both a narrowband bradycardia filterand a narrowband tachycardia filter. Briefly, at step 100, the pacer/ICDsenses electrical cardiac signals, such as IEGM signals, using one ormore leads implanted within the patient. Otherwise conventionaltechniques may be used for converting voltage signals sensed using thevarious leads into IEGM or similar signals. At step 102, the pacer/ICDselectively filters the signals using an internal narrowband bradycardiafilter and an internal narrowband tachycardia filter. By a “bradycardiafilter,” it is meant a filter configured to pass cardiac signalsappropriate for the detection of bradycardia (particularly relativelylow rate R-waves) while filtering out noise and other cardiac signals,particularly T-waves. That is, the bradycardia filter is a filteroperative to substantially eliminate signals having frequenciesassociated with ventricular repolarization events while retainingsignals having frequencies associated with at least some ventriculardepolarization events. The bradycardia filter is also referred to as a“first filter” herein. By a “tachycardia filter,” it is meant a filterconfigured to pass cardiac signals appropriate for the detection oftachyarrhythmias. That is, the tachycardia filter is a filter operativeto pass signals having frequencies associated with ventriculardepolarization events and ventricular repolarization events. Thetachycardia filter is also referred to as a “second filter” herein.Otherwise conventional bradycardia (first) and tachycardia (second)filters can be employed. In one particular example, the bradycardiafilter is configured to pass signals generally in the range of 18-118Hz. For example, the filter may be configured to have a center frequencyat 50 Hz with −3 dB points set at 18 Hz and 118 Hz. In one particularexample, the tachycardia filter is configured to pass signals generallyin the range of 10-70 Hz. For example, the filter may be configured tohave a center frequency at 30 Hz with −3 dB points set at 10 Hz and 70Hz. As already noted, a wideband filter may additionally be employed. Inone particular example, the wideband filter is configured to passsignals in the range of 1-250 Hz. For example, the filter may beconfigured to have a center frequency at 50 Hz with −3 dB points set at1 Hz and 250 Hz.

As will be apparent with reference to the illustrative examplesdescribed in detail below, selective filtering of the electrical cardiacsignals can include, at least: (1) filtering initial cardiac signalswith just the bradycardia filter and then filtering additional cardiacsignals using the tachycardia filter; (2) filtering initial cardiacsignals with just the bradycardia filter and then filtering additionalcardiac signals using both the bradycardia filter and the tachycardiafilter operating in parallel; (3) filtering initial cardiac signals withjust the tachycardia filter and then filtering additional cardiacsignals using both the bradycardia filter and the tachycardia filteroperating in parallel; (4) filtering initial signals using both thebradycardia filter and the tachycardia filter operating in parallel; (5)or some combination of the foregoing. Other selective filteringcombinations may be appropriate as well, depending upon theimplementation, including combinations including one r more widebandfilters.

In some implementations, physically separate bradycardia and tachycardiafilters are used. In other implementations, a single reconfigurablefilter is used, which can be selectively switched by the pacer/ICD frombradycardia filtering to tachycardia filtering. The illustrativeexamples described herein do not specifically provide for the filteringof the same cardiac signal in series by both a bradycardia filter and atachycardia filter (i.e. feeding the output of a bradycardia filter intoa tachycardia filter, or vice versa). Although, depending upon thebandwidth characteristics of the filters, such sequential filtering ofsignals may potentially be appropriate or advantageous in some cases,and hence the term “selective filtering” should be construed asencompassing such as sequential filtering embodiments.

At step 104, the pacer/ICD detects tachyarrhythmia within the patient,if it is occurring, using signals filtered by the bradycardia filter incombination with signals filtered by the tachycardia filter, and, insome cases, in further combination with wideband filtered signals.Various exemplary techniques for detecting ventricular tachyarrhythmiasare set forth below in the various illustrative examples. Principles ofthe invention may potentially be applied to the detection of atrialtachyarrhythmias as well as ventricular tachyarrhythmias, or to otheratrial or ventricular arrhythmias or dysrhythmias, as well. At step 106,the pacer/ICD delivers therapy in response to the detectedtachyarrhythmia, such as ATP in response to VT or defibrillation shocksin response to VF. ATP is discussed in, e.g., U.S. Pat. No. 6,907,286 toKroll, et al., entitled “Anti-tachycardia Pacing Methods and Devices.”Defibrillation therapy is discussed in, e.g., U.S. Pat. No. 6,772,007 toKroll, entitled “System and Method of Generating a Low-Pain Multi-StepDefibrillation Waveform for Use in an ImplantableCardioverter/Defibrillator (ICD).”

Note that, whereas the techniques of FIG. 5 are preferably employed in“real time” based on IEGM signals as they are sensed, the techniques canalternatively be employed based on previously recorded signals. Forexample, IEGM data may be collected over time then analyzed later todetect episodes of arrhythmias that may have already occurred for thepurpose of generate appropriate diagnostic data for physician review.Such delayed analysis techniques can be performed either using theimplanted device itself or using an external data processing devicebased on data transmitted from the implanted device. Real time detectionis preferred as it allows arrhythmias to be promptly detected so thatappropriate therapy can be promptly delivered. In the examples below,ventricular tachyarrhythmias are detected though, as noted, principlesof the invention are potentially applicable to the detection of atrialtachyarrhythmias as well.

First Exemplary Ventricular Tachyarrhythmia Detection Technique

FIG. 6 illustrates a first exemplary technique, wherein the bradycardiafilter is used to provide a preliminary indication of ventriculartachyarrhythmia, which is then confirmed using the tachycardia filter.At step 200, the pacer/ICD detects a preliminary indication ofventricular tachyarrhythmia using signals filtered by the bradycardiafilter. That is, ventricular channel electrical cardiac signals sensed,e.g., between an RV tip electrode and the device housing or between theRV tip electrode and an LV tip electrode, are filtered by thebradycardia filter and then analyzed to determine whether there is asignificant likelihood that a ventricular tachycardia is occurring.Specific techniques for use at step 200 to detect the preliminaryindication are illustrated in FIG. 7 and will be discussed below. If apreliminary indication is detected, then, at step 202, pacer/ICDactivates a tachycardia filter or, if a reconfigurable filter isprovided, the pacer/ICD switches the reconfigurable filter from abradycardia-filtering mode to a tachycardia-filtering mode. At step 204,the pacer/ICD then confirms the detection of the ventriculartachyarrhythmia using signals filtered by the tachycardia filter. Thatis, additional ventricular channel signals are filtered by thetachycardia filter and then analyzed to determine whether theventricular tachycardia is, indeed, occurring. Specific techniques foruse in confirming ventricular tachyarrhythmia at step 204 areillustrated in FIG. 8 and will be discussed below. Assuming thearrhythmia is confirmed then, at step 206, appropriate therapy isdelivered.

Although not shown, the pacer/ICD, at step 206, preferably determinesthe particular ventricular tachyarrhythmia, e.g., VT or VF and thendelivers therapy appropriate to the arrhythmia. Otherwise conventionaltechniques for distinguishing among different types of ventriculartachyarrhythmias may be employed. For example, the ventricular rate canbe compared against separate VT and VF thresholds. If the rate exceeds ahigher VF threshold (set, e.g., to 220 beats per minute (bpm)), then VFis presumed and defibrillation shocks are delivered. If the rate onlyexceeds the lower VT threshold (set, e.g., to 180 bpm), then VT ispresumed and ATP is delivered. More sophisticated discriminationtechniques may be employed as well. See, for example, U.S. Pat. No.5,404,880 to Throne, entitled “Scatter Diagram Analysis System andMethod for Discriminating Ventricular Tachyarrhythmias.” Note that, upon detection of the preliminary indication of a ventriculartachyarrhythmia at step 200, defibrillation capacitors may bepre-charged so that, if VF is subsequently detected, defibrillationshocks can be more promptly delivered.

FIG. 7 illustrates techniques for detecting a preliminary indication ofventricular tachyarrhythmias using signals filtered by the bradycardiafilter. At step 208, the pacer/ICD determines a ventricular rate basedon the ventricular channel signals filtered by the bradycardia filter,i.e. the device calculates a “bradycardia filter-based ventricularrate.” At step 210, the pacer/ICD analyzes the filtered signals todetect a preliminary indication of ventricular tachyarrhythmia bydetecting one or more of:

-   -   (a) the bradycardia filter-based ventricular rate exceeding a        predetermined VT detection threshold;

(b) a significant number of R-waves of irregular shape;

(c) a significant number of R-waves of irregular size;

(d) a significant number of R-waves occurring at a rate below the VTdetection threshold but above a rate consistent with normal sinusrhythm; and/or

(e) a lack of R-waves, wherein the lack of R-waves is not consistentwith bradycardia.

Now considering these conditions individually, insofar as (a) isconcerned, the typical bradycardia filter (assuming its sensitivity andother parameters are set properly) will accurately detect “well formed”R-waves occurring, even those occurring at VT rates. By “well formed,”it is meant that the R-waves have relatively normal morphology, i.e.they are not significantly distorted and are not fused with otherventricular events. (Irregular R-waves may or may not be detected,depending upon their shape and magnitude.) Accordingly, the bradycardiafilter-based ventricular rate can be compared against the VT threshold(e.g. 180 bpm) to detect the preliminary indication of ventriculartachyarrhythmia. The resulting indication of ventricular tachyarrhythmiais preliminary only and, as noted, confirmation is performed using thetachycardia filter before any therapy is actually delivered. Note, also,that the typical bradycardia filter will not detect R-waves associatedwith VF, as such events are usually too fast or are poorly formed.

Insofar as (b) is concerned, a preliminary indication of a ventriculartachyarrhythmia is generated if there are a significant number ofR-waves of irregular shape. In this regard, although the bradycardiafilter will not accurately detect all non-“well formed” R-waves, it maynevertheless detect some, and the presence of a significant number ofirregular shape R-waves is an indication of a possible tachyarrhythmia.Hence, a counter is used to count the number of such irregularly shapedR-waves and, if the count exceeds some predetermined threshold (e.g. Xout Y R-waves have irregular shapes, where X and Y are programmablevalues), then the preliminary indication of a ventriculartachyarrhythmia is generated. Otherwise conventional morphologicalanalysis techniques may be used to examine the R-waves to distinguish“well formed” R-waves from irregular R-waves.

Insofar as (c) is concerned, a preliminary indication of a ventriculartachyarrhythmia is generated if there are a significant number ofR-waves of irregular size. In this regard, the presence of a significantnumber of R-waves that are either much larger or much smaller than theaverage is an indication of a possible tachyarrhythmia. Hence, a counteris used to count the number of such irregularly sized R-waves and, ifthe count exceeds some predetermined threshold (e.g. X out Y R-waveshave irregularly sizes), then the preliminary indication of aventricular tachyarrhythmia is generated. Otherwise conventionalamplitude measurement techniques may be used to identify irregularlysized R-waves.

Insofar as (d) is concerned, the preliminary indication is generated ifthere are a significant number of R-waves occurring at a rate below theVT detection threshold but above a rate consistent with normal sinusrhythm. To account for the possibility that some R-waves during VT mightnot be detected by the bradycardia filter because they are not wellformed or because the filter parameters are not set properly to detecthigh rate R-waves, it is appropriate to define a somewhat lower“ventricular tachyarrhythmia preliminary detection threshold”, i.e. athreshold somewhat lower than the 180 bpm VT threshold. In one example,the lower threshold is set to, e.g., 160 bpm. Accordingly, a preliminaryindication of ventricular tachyarrhythmia may be generated whenever asignificant number of R-waves (i.e. X out of Y, where X and Y areprogrammable) exceed the “ventricular tachyarrhythmia preliminarydetection threshold.”

Insofar as (e) is concerned, a preliminary indication of a ventriculartachyarrhythmia is generated if there is a significant lack of R-waves,wherein the lack of R-waves is not consistent with bradycardia. Asnoted, during VF, R-waves are not typically detected by the bradycardiafilter. Accordingly, a lack of R-waves may be indicative of bradycardiaor VF. To determine whether the lack of R-waves is not consistent withbradycardia, the pacer/ICD may, for example, examine thebradycardia-filtered rate just prior to the period when R-waves nolonger appear. If the rate was rapidly increasing toward the VTthreshold, such would not be consistent with bradycardia, and thepreliminary indication of ventricular tachyarrhythmia would begenerated. If the rate was dropping from a normal sinus rhythm rate,such would be consistent with bradycardia, and so no indication ofventricular tachyarrhythmia would be generated. (Instead, an indicationof bradycardia would be generated and appropriate bradycardia therapydelivered.)

Hence, FIG. 7 illustrates some techniques for rendering a preliminarydetermination of ventricular tachyarrhythmia based on signals filteredwith a bradycardia filter. Other techniques may be appropriate as well.Multiple techniques may be employed in combination.

FIG. 8 illustrates an exemplary technique for confirming the detectionof a ventricular tachyarrhythmia using signals filtered by thetachycardia filter. At step 212, the pacer/ICD determines a ventricularrate based on the signals filtered by the tachycardia filter, i.e. thepacer/ICD calculates a “tachycardia filter-based ventricular rate.” Atstep, 214, the pacer/ICD verifies that the ventricular rate exceeds, atleast, the VT detection threshold. If so, then the ventriculartachyarrhythmia is confirmed at step 216. Otherwise, it is disconfirmed,at step 218. As already explained, before any therapy is actuallydelivered, the pacer/ICD distinguishes between VT and VF, typically beemploying a still higher VF threshold. FIG. 8 merely illustrates oneexemplary technique for confirming ventricular tachyarrhythmia basedusing signals filtered by the tachycardia filter. Other techniques mayadditionally or alternatively be employed.

Second Exemplary Ventricular Tachyarrhythmia Detection Technique

FIG. 9 illustrates the second exemplary technique, wherein signalsfiltered by the bradycardia filter are compared with signals filtered bythe tachycardia filter to detect tachyarrhythmia, i.e. the bradycardiaand the tachycardia filter operate in parallel. Since the filtersoperate in parallel, a reconfigurable filter of the type described aboveis not used. Rather, separate bradycardia and tachycardia filters areemployed. At step 300, the pacer/ICD filters ventricular channel signalssensed via the leads using the tachycardia filter while, at step 302,also filtering ventricular channel signals sensed via the leads usingthe bradycardia filter. At step 304, the pacer/ICD compares theventricular 0channel signals filtered by the tachycardia filter and thebradycardia filter to detect ventricular tachyarrhythmia. Assuming aventricular arrhythmia is detected then, at step 306, appropriatetherapy is delivered.

FIG. 10 illustrates techniques for detecting ventricular tachyarrhythmiabased on a comparison between bradycardia-filtered signals andtachycardia-filter signals for use at step 304 of FIG. 9. At step 308,the pacer/ICD determines a “tachycardia filter-based ventricular rate”from the signals filtered by the tachycardia filter while alsodetermining a “bradycardia filter-based ventricular rate” from thesignals filtered by the bradycardia filter. At step 310, the pacer/ICDcompares the tachycardia filter-based ventricular rate to VT detectionthreshold. If the tachycardia filter-based rate does not exceed thethreshold, then there is no indication of ventricular tachyarrhythmia,step 312. However, T-wave oversensing may be occurring and so furtherprocessing is performed, which will be described with reference to FIG.11. If, however, the tachycardia filter-based rate exceeds the VTthreshold, then, at step 314, the pacer/ICD compares the tachycardiafilter-based ventricular rate to twice the bradycardia filter-basedventricular rate. If the tachycardia filter-based ventricular rate isgreater than twice the bradycardia filter-based ventricular rate, thenventricular tachyarrhythmia is thereby detected without furtherprocessing (i.e. T-wave oversensing is not implicated.) If, however, thetachycardia filter-based ventricular rate is not greater than twice thebradycardia filter-based ventricular rate, then a ventriculartachyarrhythmia might be occurring or the high rate might be due toT-wave oversensing and so further processing is required, which willalso be described with reference to FIG. 11.

In other words, if the tachycardia filter-based ventricular rate isgreater than the VT threshold and if the tachycardia filter-basedventricular rate is also greater than twice the bradycardia filter-basedventricular rate, a tachyarrhythmia is immediately detected (block 316)without the need for further processing. In the regard, tachycardia isalmost certainly occurring, since T-wave oversensing, by itself, wouldnot produce such a result. T-wave oversensing results in, at most, atachycardia-filtered rate that is twice the bradycardia-filtered rate(i.e. each T-wave is misidentified as an R-wave.) If, instead, thetachycardia filter-based ventricular rate is greater than the VTthreshold but not greater than twice the bradycardia filter-basedventricular rate (block 318), then additional confirmation proceduresare needed to verify the tachyarrhythmia before therapy is delivered.

Accordingly, at step 320 of FIG. 11, the pacer/ICD initiates additionalVT/VF confirmation procedures, i.e. detection procedures not basedsolely on the tachycardia rate and the VT and VF thresholds. Techniquesdescribed below with reference to FIGS. 13-19, wherein the pacer/ICDdistinguishes between true and false R-waves, may be appropriate for useat step 320. Also, morphology-based detection VT/VF techniques may beused. See, also, U.S. Pat. No. 5,623,936 to McClure, entitled“Implantable Medical Device having Means for Discriminating between TrueR-Waves and Ventricular Fibrillation.” If VT/VF is confirmed by thealternative techniques, then processing returns to FIG. 9 and therapy ispromptly delivered. Otherwise, the high tachycardia rate was deemed tobe due to T-wave oversensing and sensitivity of the tachycardia filteris adjusted at step 322 in an attempt to eliminate or reduceoversensing, such as by decreasing the sensitivity of the tachycardiafilter.

If, at step 312 of FIG. 10, the tachycardia filter-based ventricularrate was not greater than the VT threshold, then there is no indicationof an arrhythmia, but T-wave oversensing may be occurring and so furtherprocessing is warranted. Accordingly, at step 324 of FIG. 11, thepacer/ICD compares the tachycardia filter-based ventricular rate totwice the bradycardia filter-based ventricular rate and if it is foundto be about equal to twice the bradycardia filter-based ventricularrate, T-wave oversensing is thereby detected, step 326, and thesensitivity of the tachycardia filter is adjusted at step 328. In thisregard, if the ventricular rate derived from the tachycardia filter isabout equal to twice the ventricular rate derived bradycardia filter,but below the VT threshold, the tachycardia filter rate is likely due toT-wave oversensing, i.e. each T-wave is being misidentified as anR-wave, yielding a rate double that of the bradycardia filter.Otherwise, if the tachycardia-filtered rate is not about equal to twicethe bradycardia rate, then no T-wave oversensing is detected, step 328,and processing merely returns to FIG. 9.

The logic of FIGS. 9-11 is summarized in Table I, above. As furtherindicated in the table, if the rate derived from the tachycardia filteris well below the VT threshold and is also about equal to the ratederived from the bradycardia filter, then normal sinus rhythm isoccurring without T-wave oversensing and so no action need be taken.

Third Exemplary Ventricular Tachyarrhythmia Detection Technique

FIG. 12 illustrates the third exemplary technique, wherein a preliminaryindication of tachyarrhythmia is made using the tachycardia filter andthen additional signals filtered by the bradycardia filter are comparedwith additional signals filtered by the tachycardia filter to confirmdetection of the tachyarrhythmia. At step 400, the pacer/ICD detects apreliminary indication of ventricular tachyarrhythmia using signalsfiltered by the tachycardia filter by comparing tachycardia filter rateagainst VT threshold. See, for example, FIG. 8. If a preliminaryindication is detected, then the pacer/ICD proceeds to confirm thetachyarrhythmia by using the bradycardia filter in combination with thetachycardia filter. Accordingly, at step 402, the pacer/ICD activatesthe bradycardia filter for tachyarrhythmia detection purposes. In thisregard, the bradycardia filter may already be operating to detectbradycardia, in which case the pacer/ICD just begins routing outputsignals from the bradycardia filter to the tachyarrhythmia detectionsystem. If not already active, the pacer/ICD activates the bradycardiafilter to beginning filtering ventricular channel signals. In any case,at step 404, the pace/ICD confirms the detection of tachyarrhythmia bycomparing additional ventricular channel signals filtered by thetachycardia filter and by the bradycardia filter. The techniques justdescribed with reference to FIGS. 10 and 11 may be used. Assuming thetachyarrhythmia is confirmed then, at step 406, appropriate therapy isdelivered.

Hence, the embodiment of FIG. 12 is similar to that of FIGS. 9-11,described above, but with FIG. 12 the tachycardia filter is initiallyused to detect a preliminary indication of tachyarrhythmia before anybradycardia-filtered signals are compared against tachycardia-filteredsignals. The preliminary indication may be used, e.g., to triggercharging of defibrillation capacitors in the case that a defibrillationshock is ultimately required.

Fourth Exemplary Ventricular Tachyarrhythmia Detection Technique

FIG. 13 summarizes the fourth exemplary technique, wherein a preliminaryindication of tachyarrhythmia is made using the tachycardia filter andthen additional signals filtered by the bradycardia filter are comparedwith additional signals filtered by the tachycardia filter to identifytrue R-waves (as opposed to oversensed T-waves) so as to permitdetection of tachyarrhythmia based only on true R-waves. At step 500,the pacer/ICD detects a preliminary indication of ventriculartachyarrhythmia using signals filtered by the tachycardia filter bycomparing tachycardia filter rate against VT threshold. See, forexample, FIG. 8. If a preliminary indication is detected, then thepacer/ICD initiates a confirmation period or confirmation intervalduring which the device seeks to confirm the tachyarrhythmia beforetherapy is delivered. The confirmation period may extend, e.g., for 100ventricular event cycles following the preliminary detection at step500. Alternatively, the confirmation period may be specified as apredetermined number of seconds, such as 30 seconds, or as the lesser ofa predetermined number of ventricular event cycles or a predeterminednumber of seconds. In any case, during the confirmation period, steps502 and 504 are performed wherein the pacer/ICD filters additionalventricular channel signals, in parallel, using both the bradycardiafilter and the tachycardia filter.

At step 506, the pacer/ICD compares the ventricular channel signalsfiltered by the bradycardia filter with the ventricular channel signalsfiltered by the tachycardia filter to distinguish between true R-wavesand false R-waves (i.e. oversensing T-waves). Techniques set forth inFIG. 14, described below, may be employed. Following completion of theconfirmation period, the pacer/ICD then detects and/or confirms theventricular tachyarrhythmia, at step 508, based on the true R-waves, andonly the true R-waves, detected during the confirmation period. That is,oversensed T-waves occurring during the confirmation period are ignoredfor the purposes of confirmation of the ventricular tachyarrhythmia. Inone example, the pacer/ICD calculates a “true ventricular rate” usingonly the true R-waves and compares the true ventricular rate against theVT threshold to confirm the tachyarrhythmia. Assuming thetachyarrhythmia is confirmed then, at step 510, appropriate therapy isdelivered. If not, then the preliminary indication of tachyarrhythmiamade at step 500 was likely due to oversensed T-waves and so the device,at step 512, adjusts the sensitivity of the tachycardia filter in anattempt to reduce or eliminate such oversensing.

Turning now to FIG. 14, an exemplary technique for distinguishing trueand false R-waves will be described for use at step 506 of FIG. 13. Atstep 514, the pacer/ICD detects a first event in either thebradycardia-filtered signals or in the tachycardia-filtered signals. Insome cases, that event may appear first in the bradycardia-filteredsignal. In other cases, it may appear first in the tachycardia-filteredsignal. In still other cases, the event may appear substantiallysimultaneously in both signals. Next, at step 516, the pacer/ICDdetermines whether another (i.e. a second) event is detected in thetachycardia filtered signals within a predetermined time windowfollowing the first event (e.g. within 50-150 milliseconds (ms)). If asecond event is detected in the tachycardia-filtered signals within thewindow, then the first event is deemed to be a true R-wave whereas thesecond event is deemed to be an oversensed T-wave and is ignored for thepurposes of ventricular rate calculation. In other words, if a pair ofconsecutive events is separated by less than the window interval, thenthe second of the pair of events is regarded as being an oversensedT-wave and is ignored for the purposes of the ventricular ratecalculation. The first event of the pair, however, is counted.Processing then returns to step 514, wherein the pacer/ICD waits todetect another event in either the bradycardia and/or thetachycardia-filtered signals.

If, however, the second event is not detected in thetachycardia-filtered signals until after the end of the time window,then both the first and second events of the pair of events are deemedto be true R-waves. In other words, if a pair of consecutive events isseparated by more than the window interval, then the two events are bothregarded as being true R-waves. Processing then returns to step 516,wherein the pacer/ICD waits to detect another event in thetachycardia-filtered signals. Note that, following step 518, processingreturns to step 514; whereas, following step 520, processing returns tostep 516. In this regard, following step 518, since the second event ofthe pair of events was rejected as being a T-wave, the next event to bedetected on either the bradycardia or tachycardia channels will likelybe the next true R-wave and is hence should be regarded as the firstevent of the next pair of events. Thus, further processing at step 514is appropriate. However, following step 520, since the second event ofthe pair of events was deemed to be a true R-wave, that second event canbe regarded as the first event of the next pair of events, and hencefurther processing at step 516 is appropriate.

Turning now to FIG. 15, exemplary techniques for confirmingtachyarrhythmia based on true R-waves will be described for use at step508 of FIG. 13. At step 522, a first illustrative technique begins,wherein the pacer/ICD determines a true ventricular rate based only onthe true R-waves. Then, at step 524, the pacer/ICD compares the trueventricular rate against the VT threshold and, if the rate exceeds thethreshold, tachyarrhythmia is confirmed, at step 526. Otherwise,tachyarrhythmia is disconfirmed, at step 528. Thus, the firstillustrative confirmation technique uses only the true R-waves. Thesecond illustrative technique 509′ of FIG. 15 instead examines both trueand false R-waves. That is, beginning at step 530, the pacer/ICD countsthe number of false R-waves within a predetermined number of combinedfalse and true R-waves (e.g. X true R-waves out of Y total R-waves.)Then, at step 532, the pacer/ICD compares the count to a predeterminedcount threshold indicative of tachycardia filter oversensing (e.g. arethere at least seven false counts out of each group of 10 total counts?)If so, then the tachyarrhythmia is disconfirmed, at step 534. That is,the majority of R-waves are false R-waves and hence significant T-waveoversensing is occurring. As such, the high ventricular rate initiallydetected at step 500 of FIG. 13 was likely due to T-wave oversensing andwas not indicative of a true ventricular tachyarrhythmia. If however,the count of false R-waves does not exceed the count threshold, thetachyarrhythmia is confirmed, at step 536. That is, the majority ofR-waves are true R-waves and so the high ventricular rate initiallydetected at step 500 of FIG. 13 is indicative of a true ventriculartachyarrhythmia.

Hence, the embodiment of FIGS. 13-15 uses the tachycardia filter todetect a preliminary indication of tachyarrhythmia then comparestachycardia and bradycardia filtered signals to distinguish true R-wavesfrom false R-waves and to then confirm or disconfirm the tachyarrhythmiabased on the true R-waves.

Fifth Exemplary Ventricular Tachyarrhythmia Detection Technique

FIG. 16 summarizes the fifth exemplary technique, wherein signalsfiltered by the bradycardia filter, the tachycardia filter and awideband filter are compared to identify true R-waves so as to permitdetection of tachyarrhythmia based only on true R-waves. Thus, with thetechnique of FIG. 16 a wideband filter is also employed. Beginning atsteps 600, 602 and 604, the pacer/ICD simultaneously filters ventricularchannel cardiac signals using the tachycardia filter, the bradycardiafilter and the wideband filter, respectively. Then, at steps 606, 608and 610, the pacer/ICD detects possible R-waves within the signalsfiltered by the tachycardia filter, the bradycardia filter and thewideband filter, respectively. At step 612, the pacer/ICD compares thetiming of the various ventricular events to identify true R-waves and tocalculate true ventricular rate. Techniques for use at step 612 areillustrated in FIG. 18, to be discussed below. Then, at step 614, thepacer/ICD detects tachyarrhythmia based on true ventricular rate derivedfrom the true R-waves and, at step 616, delivers appropriate therapy.

Turning now to FIG. 17, an exemplary technique for identifying trueR-waves will be described for use at step 612 of FIG. 16. At step 618,the pacer/ICD detects circumstances wherein an event is detected at thesame time within signals filtered by the wideband filter, thebradycardia filter, and the tachycardia filter. That is, an event isdetected substantially simultaneously using all three filters. In thiscase, the event is regarded as being a true R-wave, at step 620, and iscounted toward the ventricular rate at step 622. In this regard, for anevent to appear on all three filtered signals at the same time, theevent almost certainly must be a true R-wave. If it were another event,such as a T-wave, it would not have appeared on the bradycardia-filteredsignal. Meanwhile, at step 624, the pacer/ICD detects circumstanceswherein an event is detected at the same time within signals filtered bythe wideband filter and the tachycardia filter but not the bradycardiafilter. In this case, the event is regarded, at step 626, as being ananomalous event indicative of: (a) a possible T-wave oversensed on thetachycardia filter or (b) a possible R-wave occurring during VF. In thisregard, for an event to appear on the wideband and tachycardia filteredsignals at the same time but not on the bradycardia filtered signals, itis either a T-wave that is being oversensed by the tachycardia (whilebeing properly filtered out by the bradycardia filter), or it is a veryhigh rate VF R-wave that the tachycardia filter is detecting but that isat too high a rate for the bradycardia filter to detect. Step 628 isperformed to distinguish (a) from (b). The techniques of FIG. 18,discussed below, maybe employed at step 628.

Still further, at step 630, the pacer/ICD detects circumstances whereinan event is detected at the same time within signals filtered by thewideband filter and the bradycardia filter but not the tachycardiafilter. In this case, the event is regarded, at step 632, as being ananomalous event indicative of: a true R-wave not detected withtachycardia filter due to under-sensing by that filter. In this regard,for an event to appear on the wideband and bradycardia filtered signalsat the same time, it must be a low rate R-wave. Low rate R-waves shouldalso be detected by the tachycardia filter. Hence, if the low rateR-wave is not detected on the tachycardia filter, it is likely due toundersensing by that filter. That is, the sensitivity of the tachycardiafilter is set to low. Accordingly, at step 634, the pacer/ICD counts theevent toward ventricular rate and adjusts tachycardia filter in anattempt to reduce or eliminate the tachycardia filter undersensing.

Turning now to FIG. 18, an exemplary technique is provided fordistinguishing T-waves oversensed on the tachycardia filter from R-wavesoccurring during VF, for use at step 628 of FIG. 17. As noted, thisprocessing is triggered if an event is detected at the same time by thewideband filter and by tachycardia filter but not by the bradycardiafilter. At step 636, the pacer/ICD determines a ventricular rate fromthe wideband filtered signals only and then determines, at step 638,whether the wideband ventricular rate is consistent with VF. To be ableto determine whether the wideband ventricular rate is consistent withVF, the pacer/ICD may, for example, periodically track thewideband-filtered rate and record that information. The pacer/ICDexamines recent values of the wideband-filtered rate and, if thewideband-filtered rate had recently increasing very rapidly, then suchwould be consistent with VF. (Note that a high wideband rate, by itself,is not necessarily indicative of VF, since the high rate might be due tosubstantial oversensing within the wideband-filtered signals.)

In any case, if the wideband-filtered rate is consistent with VF, thenthe anomalous event of step 626 of FIG. 17 is likely an R-wave occurringduring VF, i.e. an R-wave occurring at a rate too high for thebradycardia filter to detect. Accordingly, at step 640, the pacer/ICDidentifies the anomalous event as being an R-wave occurring duringpossible VF and is counted toward the ventricular rate. At step 642, thebradycardia-filtered signals are ignored and, at step 644, the possibleVF is verified using only tachycardia filtered signals. For example, aventricular rate derived exclusively from tachycardia-filtered signalsmay be compared against a VF rate threshold. If it exceeds thethreshold, VF is verified and one or more defibrillation shocks areimmediately delivered in an effort to revert the heart to a normal sinusrhythm, at step 646. If VF is not verified, it is still possible that VTis occurring and so processing returns to FIG. 17 and hence to FIG. 16so that the ventricular rate can be compared against a VT threshold(step 614) and appropriate VT therapy delivered (step 616.)

On the other hand, if the wideband-filtered rate determined at step 636is not consistent with VF, then the anomalous event of step 626 of FIG.17 is likely the result of T-wave oversensing, i.e. a T-wave beingerroneously detected by the tachycardia filter. Accordingly, at step648, the pacer/ICD identifies the anomalous event as being an oversensedT-wave. The event is not counted toward ventricular rate. The pacer/ICDpreferably adjusts the tachycardia filter sensitivity so as to preventor reduce further T-wave oversensing. Even in the presence of T-waveoversensing, it is still possible that VT is occurring and so processinglikewise returns to FIG. 17 and hence to FIG. 16 so that the ventricularrate can be compared against the VT threshold (step 614) and appropriateVT therapy can be delivered (step 616.)

The logic of FIGS. 16-18 is summarized in Table II, above. Although notshown in the figures, any event not detected using either thebradycardia filter or the tachycardia filter, but which is detectedusing the wideband filter is noise or is a far-field P-wave, and isignored for the purposes of ventricular rate calculation. Likewise, inthe unlikely event that an event is detected on both the bradycardia andtachycardia filters but not on the wideband filter, that event isignored as an anomalous event, likely arising due to noise on thebradycardia and tachycardia channels.

Thus, FIGS. 5-18 illustrate various techniques for detecting ventriculartachyarrhythmia. Depending up on the implementation, the techniques maybe implemented separately or, in some cases, may be implementedtogether. That is, pacer/ICDs may be provided that combine two or moreof the exemplary techniques for use in the detection and confirmation oftachyarrhythmias. For example, in some implementations, therapy isdelivered if any of the various techniques detect and confirmventricular tachyarrhythmia. In other implementations, therapy isdelivered only if each of the techniques detects and confirmsventricular tachyarrhythmia. As can be appreciated, a variety ofcombinations of the various techniques are available in accordance withthe invention and such combinations will not be described further.

Overview of T-Wave Oversensing Detection Techniques

Turning now to FIGS. 19-22, techniques for detecting T-wave oversensingwill be summarized, wherein the techniques use the narrowbandtachycardia filter and the narrowband bradycardia filter and, in someexamples, additionally use the wideband filter. These T-wave oversensingdetection techniques have already been described and discussed inconnection with the tachyarrhythmia detection techniques above. Itshould be understood, however, that the T-wave oversensing detectiontechniques can be used independently, i.e. the techniques need not beexploited only in furtherance of tachyarrhythmia detection. Accordingly,for the sake of completeness, FIGS. 19-22 are provided to set forth theT-wave oversensing techniques independently of the tachyarrhythmiadetection techniques. Since these techniques have already been describedand discussed, detailed descriptions will not be provided again. Rather,the techniques will only be summarized.

FIG. 19 provides a broad overview of the techniques for detecting T-waveoversensing using both bradycardia filtered signals andtachycardia-filtered signals. At step 700, the pacer/ICD senseselectrical cardiac signals using leads implanted within a patient. Atstep 702, the pacer/ICD selectively filters the signals using abradycardia filter and a tachycardia filter. At step 704, the pacer/ICDdetects the oversensing of ventricular repolarization events (T-waves)within the signals filtered by the tachycardia filter by comparing thesignals filtered by the tachycardia filter with the signals filtered bythe bradycardia filter. Preferably, if T-wave oversensing is detected,the oversensed T-waves are rejected for the purposes of calculating theventricular rate, triggering or inhibiting therapy, etc. Also,preferably, the sensitivity of the tachycardia filter is adjusted so toreduce or eliminate further T-wave oversensing.

Note that, whereas the techniques of FIG. 19 are preferably employed in“real time” based on IEGM signals as they are sensed, the techniques canalternatively be employed based on previously recorded signals. Forexample, IEGM data may be collected over time then analyzed later todetect T-wave oversensing that may have already occurred for the purposeof generate appropriate diagnostic data for physician review. Suchdelayed analysis techniques can be performed either using the implanteddevice itself or using an external data processing device based on datatransmitted from the implanted device. Real time detection is preferredas it allows T-wave oversensing to be promptly detected so thatappropriate action can be taken.

FIGS. 20-22 summarize various exemplary techniques for performing thesteps of FIG. 19.

First Exemplary T-Wave Oversensing Detection Technique

FIG. 20 illustrates a first exemplary T-wave oversensing detectiontechnique, wherein T-wave oversensing is detected if a tachycardiafilter-based ventricular rate is about twice a bradycardia filter-basedventricular rate. Beginning at step 800, the pacer/ICD filtersventricular channel signals using the bradycardia filter and determinesa bradycardia filter-based ventricular rate. Concurrently, at step 802,the pacer/ICD filters ventricular channel signals using the tachycardiafilter and determines a tachycardia filter-based ventricular rate. Atstep 804, the pacer/ICD compares the tachycardia filter-basedventricular rate to the bradycardia filter-based ventricular rate. Atstep 806, the pacer/ICD detects oversensing of T-waves within signalsfiltered by the tachycardia filter by determining if the tachycardiafilter-based ventricular rate is about twice the bradycardiafilter-based ventricular rate. For further descriptions of this T-waveoversensing detection technique see, e.g., Table I above, as well as thedescriptions of FIGS. 9-11.

Second Exemplary T-Wave Oversensing Detection Technique

FIG. 21 illustrates the second exemplary T-wave oversensing detectiontechnique, wherein a detection window is employed to detect T-waveoversensing. Beginning at step 900, the pacer/ICD filters ventricularchannel signals using the tachycardia filter and identifies ventricularevents therein. Concurrently, at step 904, the pacer/ICD filtersventricular channel signals using the bradycardia filter and identifiesventricular events therein. Upon detection of a first ventricular eventeither in the signals filtered by the bradycardia filter or in thesignals filtered by the tachycardia filter, the pacer/ICD, at step 906,determines whether a second ventricular event is detected in the signalsfiltered by the tachycardia filter within a predetermined time windowfollowing the first event. If so, then, at step 906, the pacer/ICDidentifies the second event as being a false R-wave indicative oftachycardia-filter oversensing. If not, then, at step 908, the pacer/ICDidentifies the second event as being a true R-wave. For furtherdescriptions of this T-wave oversensing detection technique see, e.g.,the descriptions of FIG. 14.

Third Exemplary T-Wave Oversensing Detection Technique

FIG. 22 illustrates the third exemplary T-wave oversensing detectiontechnique, wherein signals filtered by a bradycardia filter, atachycardia filter and a wideband filter are compared to detect T-waveoversensing. Beginning at step 1000, the pacer/ICD identifies possibleR-waves within signals filtered, respectively, by the wideband filter,the bradycardia filter, and the tachycardia filter. At step 1002, thepacer/ICD then compares the timing of the possible R-waves identifiedwithin the respective filtered signals to identify oversensed T-waves.For further descriptions of this T-wave oversensing detection technique,see, e.g., Table II above, as well as the descriptions of FIGS. 16-18.

Thus, FIGS. 19-22 illustrate various techniques for detecting T-waveoversensing. Depending up on the implementation, the techniques may beimplemented separately or, in some cases, may be implemented together.That is, pacer/ICDs may be provided that combine two or more of theexemplary techniques for use in the detection of T-wave oversensing. Ascan be appreciated, a variety of combinations of the various techniquesare available in accordance with the invention and such combinationswill not be described further.

The various techniques discussed above may be implemented in any of avariety of implantable medical devices. For the sake of completeness, adetailed description of an exemplary pacer/ICD for performing thesetechniques will now be provided. However, principles of invention may beimplemented within other pacer/ICD implementations or within otherdevices.

Exemplary Pacemaker/ICD

FIG. 23 provides a simplified block diagram of the pacer/ICD, which is amulti-chamber stimulation device capable of treating both fast and slowarrhythmias with stimulation therapy, including cardioversion,defibrillation, and pacing stimulation (as well as capable of detectingT-wave oversensing, detecting tachyarrhythmias, and deliveringappropriate therapy.) To provide atrial chamber pacing stimulation andsensing, pacer/ICD 26 is shown in electrical communication with a heart1112 by way of a left atrial lead 1120 having an atrial tip electrode1122 and an atrial ring electrode 1123 implanted in the atrialappendage. Pacer/ICD 26 is also in electrical communication with theheart by way of a right ventricular lead 1130 having, in thisembodiment, a ventricular tip electrode 1132, a right ventricular ringelectrode 1134, a right ventricular (RV) coil electrode 1136, and asuperior vena cava (SVC) coil electrode 1138. Typically, the rightventricular lead 1130 is transvenously inserted into the heart so as toplace the RV coil electrode 1136 in the right ventricular apex, and theSVC coil electrode 1138 in the superior vena cava. Accordingly, theright ventricular lead is capable of receiving cardiac signals, anddelivering stimulation in the form of pacing and shock therapy to theright ventricle.

To sense left atrial and ventricular cardiac signals and to provide leftchamber pacing therapy, pacer/ICD 26 is coupled to a “coronary sinus”lead 1124 designed for placement in the “coronary sinus region” via thecoronary sinus os for positioning a distal electrode adjacent to theleft ventricle and/or additional electrode(s) adjacent to the leftatrium. As used herein, the phrase “coronary sinus region” refers to thevasculature of the left ventricle, including any portion of the coronarysinus, great cardiac vein, left marginal vein, left posteriorventricular vein, middle cardiac vein, and/or small cardiac vein or anyother cardiac vein accessible by the coronary sinus. Accordingly, anexemplary coronary sinus lead 1124 is designed to receive atrial andventricular cardiac signals and to deliver left ventricular pacingtherapy using at least a left ventricular tip electrode 1126, leftatrial pacing therapy using at least a left atrial ring electrode 1127,and shocking therapy using at least a left atrial coil electrode 1128.With this configuration, biventricular pacing can be performed. Althoughonly three leads are shown in FIG. 23, it should also be understood thatadditional stimulation leads (with one or more pacing, sensing and/orshocking electrodes) may be used in order to efficiently and effectivelyprovide pacing stimulation to the left side of the heart or atrialcardioversion and/or defibrillation.

A simplified block diagram of internal components of pacer/ICD 26 isshown in FIG. 24. While a particular pacer/ICD is shown, this is forillustration purposes only, and one of skill in the art could readilyduplicate, eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation aswell as providing for the aforementioned apnea detection and therapy.The housing 1140 for pacer/ICD 26, shown schematically in FIG. 24, isoften referred to as the “can“”, case” or “case electrode” and may beprogrammably selected to act as the return electrode for all “unipolar”modes. The housing 1140 may further be used as a return electrode aloneor in combination with one or more of the coil electrodes, 1128, 1136and 1138, for shocking purposes. The housing 1140 further includes aconnector (not shown) having a plurality of terminals, 1142, 1143, 1144,1146, 1148, 1152,1154, 1156 and 1158 (shown schematically and, forconvenience, the names of the electrodes to which they are connected areshown next to the terminals). As such, to achieve right atrial sensingand pacing, the connector includes at least a right atrial tip terminal(A_(R) TIP) 1142 adapted for connection to the atrial tip electrode 1122and a right atrial ring (A_(R) RING) electrode 1143 adapted forconnection to right atrial ring electrode 1143. To achieve left chambersensing, pacing and shocking, the connector includes at least a leftventricular tip terminal (V_(L) TIP) 1144, a left atrial ring terminal(A_(L) RING) 1146, and a left atrial shocking terminal (A_(L) COIL)1148, which are adapted for connection to the left ventricular ringelectrode 1126, the left atrial tip electrode 1127, and the left atrialcoil electrode 1128, respectively. To support right chamber sensing,pacing and shocking, the connector further includes a right ventriculartip terminal (V_(R) TIP) 1152, a right ventricular ring terminal (V_(R)RING) 1154, a right ventricular shocking terminal (R_(V) COIL) 1156, andan SVC shocking terminal (SVC COIL) 1158, which are adapted forconnection to the right ventricular tip electrode 1132, rightventricular ring electrode 1134, the RV coil electrode 1136, and the SVCcoil electrode 1138, respectively. Separate terminals (not shown) may beprovided for connecting the implanted warning device 14 and theimplanted drug pump 18, which are instead shown coupled directly tointernal functional components of the pacer/ICD that control thesedevices.

At the core of pacer/ICD 26 is a programmable microcontroller 1160,which controls the various modes of stimulation therapy. As is wellknown in the art, the microcontroller 1160 (also referred to herein as acontrol unit) typically includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling the delivery ofstimulation therapy and may further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 1160 includes the ability to process or monitorinput signals (data) as controlled by a program code stored in adesignated block of memory. The details of the design and operation ofthe microcontroller 1160 are not critical to the invention. Rather, anysuitable microcontroller 1160 may be used that carries out the functionsdescribed herein. The use of microprocessor-based control circuits forperforming timing and data analysis functions are well known in the art.

As shown in FIG. 24, an atrial pulse generator 1170 and aventricular/impedance pulse generator 1172 generate pacing stimulationpulses for delivery by the right atrial lead 1120, the right ventricularlead 1130, and/or the coronary sinus lead 1124 via an electrodeconfiguration switch 1174. It is understood that in order to providestimulation therapy in each of the four chambers of the heart, theatrial and ventricular pulse generators, 1170 and 1172, may includededicated, independent pulse generators, multiplexed pulse generators orshared pulse generators. The pulse generators, 1170 and 1172, arecontrolled by the microcontroller 1160 via appropriate control signals,1176 and 1178, respectively, to trigger or inhibit the stimulationpulses.

The microcontroller 1160 further includes timing control circuitry (notseparately shown) used to control the timing of such stimulation pulses(e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction(A-A) delay, or ventricular interconduction (V-V) delay, etc.) as wellas to keep track of the timing of refractory periods, blankingintervals, noise detection windows, evoked response windows, alertintervals, marker channel timing, etc., which is well known in the art.Switch 1174 includes a plurality of switches for connecting the desiredelectrodes to the appropriate I/O circuits, thereby providing completeelectrode programmability. Accordingly, the switch 1174, in response toa control signal 1180 from the microcontroller 1160, determines thepolarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar,etc.) by selectively closing the appropriate combination of switches(not shown) as is known in the art.

Atrial sensing circuits 1182 and ventricular sensing circuits 1184 mayalso be selectively coupled to the right atrial lead 1120, coronarysinus lead 1124, and the right ventricular lead 1130, through the switch1174 for detecting the presence of cardiac activity in each of the fourchambers of the heart. Accordingly, the atrial (ATR. SENSE) andventricular (VTR. SENSE) sensing circuits, 1182 and 1184, may includededicated sense amplifiers, multiplexed amplifiers or shared amplifiers.The switch 1174 determines the “sensing polarity” of the cardiac signalby selectively closing the appropriate switches, as is also known in theart. In this way, the clinician may program the sensing polarityindependent of the stimulation polarity. Each sensing circuit, 1182 and1184, preferably employs one or more low power, precision amplifierswith programmable gain and/or automatic gain control and/or automaticsensitivity control, bandpass filtering, and a threshold detectioncircuit, as known in the art, to selectively sense the cardiac signal ofinterest. The outputs of the atrial and ventricular sensing circuits,1182 and 1184, are connected to the microcontroller 1160 which, in turn,are able to trigger or inhibit the atrial and ventricular pulsegenerators, 1170 and 1172, respectively, in a demand fashion in responseto the absence or presence of cardiac activity in the appropriatechambers of the heart.

The ventricular sense amplifier 1184 preferably includes theaforementioned bradycardia filter, tachycardia filter and widebandfilter, shown separately in FIG. 25, discussed below.

For arrhythmia detection, pacer/ICD 26 utilizes the atrial andventricular sensing circuits, 1182 and 1184, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used in thissection “sensing” is reserved for the noting of an electrical signal,and “detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 1160 bycomparing them to a predefined rate zone limit (i.e., bradycardia,normal, atrial tachycardia, atrial fibrillation, low rate VT, high rateVT, and fibrillation rate zones) and various other characteristics(e.g., sudden onset, stability, physiologic sensors, and morphology,etc.) in order to determine the type of remedial therapy that is needed(e.g., bradycardia pacing, antitachycardia pacing, cardioversion shocksor defibrillation shocks).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 1190. The data acquisition system 1190 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device1202. The data acquisition system 1190 is coupled to the right atriallead 1120, the coronary sinus lead 1124, and the right ventricular lead1130 through the switch 1174 to sample cardiac signals across any pairof desired electrodes. The microcontroller 1160 is further coupled to amemory 1194 by a suitable data/address bus 1196, wherein theprogrammable operating parameters used by the microcontroller 1160 arestored and modified, as required, in order to customize the operation ofpacer/ICD 26 to suit the needs of a particular patient. Such operatingparameters define, for example, pacing pulse amplitude or magnitude,pulse duration, electrode polarity, rate, sensitivity, automaticfeatures, arrhythmia detection criteria, and the amplitude, waveshapeand vector of each shocking pulse to be delivered to the patient'sheart. Other pacing parameters include base rate, rest rate andcircadian base rate.

Advantageously, the operating parameters of the implantable pacer/ICD 26may be non-invasively programmed into the memory 1194 through atelemetry circuit 1200 in telemetric communication with the externaldevice 1202, such as a programmer, transtelephonic transceiver or adiagnostic system analyzer. The telemetry circuit 1200 is activated bythe microcontroller by a control signal 1206. The telemetry circuit 1200advantageously allows intracardiac electrograms and status informationrelating to the operation of pacer/ICD 26 (as contained in themicrocontroller 1160 or memory 1194) to be sent to the external device1202 through an established communication link 1204. Pacer/ICD 26further includes an accelerometer or other physiologic sensor 1208,commonly referred to as a “rate-responsive” sensor because it istypically used to adjust pacing stimulation rate according to theexercise state of the patient. However, the physiological sensor 1208may, depending upon its capabilities, further be used to detect changesin cardiac output, changes in the physiological condition of the heart,or diurnal changes in activity (e.g., detecting sleep and wake states)and to detect arousal from sleep. Accordingly, the microcontroller 1160responds by adjusting the various pacing parameters (such as rate, AVDelay, V-V Delay, etc.) at which the atrial and ventricular pulsegenerators, 1170 and 1172, generate stimulation pulses. While shown asbeing included within pacer/ICD 26, it is to be understood that thesensor 1208 may also be external to pacer/ICD 26, yet still be implantedwithin or carried by the patient. A common type of rate responsivesensor is an activity sensor incorporating an accelerometer or apiezoelectric crystal, which is mounted within the housing 1140 ofpacer/ICD 26. Other types of physiologic sensors are also known, forexample, sensors that sense the oxygen content of blood, respirationrate and/or minute ventilation, pH of blood, ventricular gradient, etc.

The pacer/ICD additionally includes a battery 1210, which providesoperating power to all of the circuits shown in FIG. 24. The battery1210 may vary depending on the capabilities of pacer/ICD 26. If thesystem only provides low voltage therapy, a lithium iodine or lithiumcopper fluoride cell may be utilized. For pacer/ICD 26, which employsshocking therapy, the battery 1210 should be capable of operating at lowcurrent drains for long periods, and then be capable of providinghigh-current pulses (for capacitor charging) when the patient requires ashock pulse. The battery 1210 should also have a predictable dischargecharacteristic so that elective replacement time can be detected.Accordingly, pacer/ICD 26 is preferably capable of high voltage therapyand batteries or other power sources appropriate for that purpose areemployed.

As further shown in FIG. 24, pacer/ICD 26 is shown as having animpedance measuring circuit 1212 which is enabled by the microcontroller1160 via a control signal 1214. Uses for an impedance measuring circuitinclude, but are not limited to, lead impedance surveillance during theacute and chronic phases for proper lead positioning or dislodgement;detecting operable electrodes and automatically switching to an operablepair if dislodgement occurs; measuring respiration or minuteventilation; measuring thoracic impedance for determining shockthresholds; detecting when the device has been implanted; measuringstroke volume; and detecting the opening of heart valves, etc. Theimpedance measuring circuit 120 is advantageously coupled to the switch124 so that any desired electrode may be used.

In the case where pacer/ICD 26 is intended to operate as an implantablecardioverter/defibrillator (ICD) device, it detects the occurrence of anarrhythmia, and automatically applies an appropriate electrical shocktherapy to the heart aimed at terminating the detected arrhythmia. Tothis end, the microcontroller 1160 further controls a shocking circuit1216 by way of a control signal 1218. The shocking circuit 1216generates shocking pulses of low (up to 0.5 joules), moderate (0.5-10joules) or high energy (11 to 40 joules), as controlled by themicrocontroller 1160. Such shocking pulses are applied to the heart ofthe patient through at least two shocking electrodes, and as shown inthis embodiment, selected from the left atrial coil electrode 1128, theRV coil electrode 1136, and/or the SVC coil electrode 1138. The housing1140 may act as an active electrode in combination with the RV electrode1136, or as part of a split electrical vector using the SVC coilelectrode 1138 or the left atrial coil electrode 1128 (i.e., using theRV electrode as a common electrode). Cardioversion shocks are generallyconsidered to be of low to moderate energy level (so as to minimize painfelt by the patient), and/or synchronized with an R-wave and/orpertaining to the treatment of tachycardia. Defibrillation shocks aregenerally of moderate to high energy level (i.e., corresponding tothresholds in the range of 11-40 joules), delivered asynchronously(since R-waves may be too disorganized), and pertaining exclusively tothe treatment of fibrillation. Accordingly, the microcontroller 1160 iscapable of controlling the synchronous or asynchronous delivery of theshocking pulses.

Microcontroller 110 also includes a combined bradycardiafilter/tachycardia filter-based arrhythmia detection system 1201operative to detect tachyarrhythmia within the patient using signalsfiltered by the bradycardia filter in combination with the signalsfiltered by the tachycardia filter, in accordance with the techniquessummarized in FIG. 5. The microcontroller also includes a combinedbradycardia filter/tachycardia filter-based oversensing detection system1203 operative to detect the oversensing of ventricular repolarizationevents within the signals filtered by the tachycardia filter bycomparing the signals filtered by the tachycardia filter with thesignals filtered by the bradycardia filter, in accordance with thetechniques summarized in FIG. 19. Therapy provided in response to anydetected arrhythmias is controlled by a therapy controller 1205.Depending upon the implementation, the various components illustratedwithin the microcontroller may be implemented as separate hardware orsoftware modules. However, the modules may be combined so as to permitsingle modules to perform multiple functions.

FIG. 25 illustrates, in block diagram form, pertinent components of theventricular sense amplifier 1184 and the combined bradycardiafilter/tachycardia filter-based arrhythmia detection system 1201 of FIG.24, and particular illustrating pertinent sub-components thereof.Briefly, ventricular sense amplifier 1184 includes the bradycardiafilter 1207, the tachycardia filter 1209 and the wideband filter 1211.The combined bradycardia filter/tachycardia filter-based arrhythmiadetection system 1201 includes various components directed to thevarious illustrative embodiments described above. It should beunderstood that the typical combined bradycardia filter/tachycardiafilter-based arrhythmia detection system will not include each of thecomponents but may be equipped, for example, with only one of the set ofcomponents corresponding to whichever particular embodiment isimplemented. Briefly, the arrhythmia detection system may include firstembodiment components including a bradycardia filter-based preliminarydetection unit 1213 operative to detect a preliminary indication oftachyarrhythmia using signals filtered by the bradycardia filter and atachycardia filter-based confirmation unit 1215 operative to confirm thedetection of tachyarrhythmia using signals filtered by the tachycardiafilter, generally in accordance with the techniques of FIGS. 6-8,discussed above. The arrhythmia detection system may additionally oralternatively include second embodiment components including acomparison unit 1217 operative to compare the ventricular channelsignals filtered by the tachycardia filter and the bradycardia filter todetect ventricular tachyarrhythmia, generally in accordance with thetechniques of FIGS. 9-11, discussed above.

The arrhythmia detection system may additionally or alternativelyinclude third embodiment components including a tachycardia filter-basedpreliminary detection unit 1219 operative to detect a preliminaryindication of tachyarrhythmia using signals filtered by the tachycardiafilter; and a bradycardia/tachycardia filter-based confirmation unit1221 operative to then confirm the detection of tachyarrhythmia usingsignals filtered by the bradycardia filter and the tachycardia filter,generally in accordance with the techniques of FIG. 12, discussed above.The arrhythmia detection system may additionally or alternativelyinclude fourth embodiment components including a tachycardiafilter-based preliminary detection unit 1223 operative to detect apreliminary indication of tachyarrhythmia using signals filtered by thebradycardia filter; and a tachycardia filter-based confirmation unit1225 operative to detect oversensing of ventricular repolarizationevents by the tachycardia filter and to confirm the detection oftachyarrhythmia only in the absence of oversensing of ventricularrepolarization events, generally in accordance with the techniques ofFIGS. 13-15, discussed above. The arrhythmia detection system mayadditionally or alternatively include fifth embodiment componentsincluding a comparison unit 1227 operative to detect tachyarrhythmiausing signals filtered by the wideband filter in combination withsignals filters by the bradycardia and tachycardia filters, generally inaccordance with the techniques of FIGS. 16-18, discussed above.

FIG. 26 illustrates, in block diagram form, pertinent components of theventricular sense amplifier 1184 and the combined bradycardiafilter/tachycardia filter-based oversensing detection system 1203 ofFIG. 24, and particular illustrating pertinent sub-components thereof.As before, the ventricular sense amplifier 1184 includes the bradycardiafilter 1207, the tachycardia filter 1209 and the wideband filter 1211.The combined bradycardia filter/tachycardia filter-based oversensingdetection system 1203 includes various components directed to thevarious illustrative embodiments described above. It should again beunderstood that the typical combined bradycardia filter/tachycardiafilter-based oversensing detection system will not include each of thecomponents but may be equipped, for example, with only one of the set ofcomponents corresponding to whichever particular embodiment isimplanted. Briefly, the oversensing detection system may include firstembodiment components including a comparison unit 1229 operative tocompare the tachycardia filter-based ventricular rate to the bradycardiafilter-based ventricular rate; and an oversensing detection unit 1231operative to detect oversensing of ventricular repolarization eventswithin signals filtered by the tachycardia filter by determining if thetachycardia filter-based ventricular rate is about twice the bradycardiafilter-based ventricular rate, generally in accordance with thetechniques of FIG. 20, discussed above.

The oversensing detection system may additionally or alternativelyinclude second embodiment components including a detection unit 1233operative, upon detection of a first ventricular event either in signalsfiltered by the bradycardia filter or in signals filtered by thetachycardia filter, to determine whether a second ventricular event isdetected in the signals filtered by the tachycardia filter within apredetermined time window following the first event; and anidentification unit 1235 operative, if a second ventricular event isdetected within the predetermined time window, to identify the secondevent as being a false ventricular depolarization event indicative oftachycardia-filter oversensing, and operative, if a second ventricularevent is not detected within the predetermined time window, to identifythe second event as being indicative of a true ventriculardepolarization event, generally in accordance with the techniques ofFIG. 21, discussed above.

The oversensing detection system may additionally or alternativelyinclude third embodiment components including an identification unit1237 operative to identify possible ventricular depolarization eventswithin signals filtered, respectively, by the wideband filter, thebradycardia filter, and the tachycardia filter, and a comparison unit1239 operative to compare the timing of the possible ventriculardepolarization events identified within the respective filtered signalsto identify oversensed ventricular repolarization events, generally inaccordance with the techniques of FIG. 22, discussed above.

What have been described are various exemplary systems and methods foruse with an implantable system controlled by a pacer or ICD. However,principles of the invention may be exploiting using other implantablesystems or in accordance with other techniques. Thus, while theinvention has been described with reference to particular exemplaryembodiments, modifications can be made thereto without departing fromthe scope of the invention. Note also that the term “including” as usedherein is intended to be inclusive, i.e. “including but not limited to”.

1. In an implantable medical device for processing electrical cardiacsignals sensed via leads implanted within the heart of a patient,wherein the device has a first filter operative to substantiallyeliminate signals having frequencies associated with ventricularrepolarization events while retaining signals having frequenciesassociated with at least some ventricular depolarization events and asecond filter operative to pass signals having frequencies associatedwith ventricular depolarization events and ventricular repolarizationevents, a method comprising: sensing electrical cardiac signals withinthe patient; selectively filtering the signals using the first filter;selectively filtering the signals using the second filter; and detectingtachyarrhythmia within the patient using signals filtered by the firstfilter in combination with signals filtered by the second filter.
 2. Themethod of claim 1 wherein detecting tachyarrhythmia using signalsfiltered by the first filter in combination with signals filtered by thesecond filter includes: detecting a preliminary indication oftachyarrhythmia using signals filtered by the first filter; and inresponse, confirming the detection of tachyarrhythmia using signalsfiltered by the second filter.
 3. The method of claim 2 whereindetecting the preliminary indication of tachyarrhythmia using signalsfiltered by the first filter includes: activating the first filter tofilter ventricular channel signals sensed via the leads; analyzing theventricular channel signals filtered by the first filter to detect thepreliminary indication of ventricular tachyarrhythmia.
 4. The method ofclaim 1 wherein detecting tachyarrhythmia within the patient includes:filtering ventricular channel signals sensed via the leads using thesecond filter while also filtering ventricular channel signals sensedvia the leads using the first filter; and comparing the ventricularchannel signals filtered by the second filter and the first filter todetect ventricular tachyarrhythmia.
 5. The method of claim 1 whereindetecting tachyarrhythmia within the patient includes: filteringventricular channel signals sensed via the leads using the secondfilter; and detecting a preliminary indication of tachyarrhythmia usingsignals filtered by the second filter; and in response, confirming thedetection of tachyarrhythmia by comparing additional signals filtered bythe second filter with additional signals filtered by the first filter.6. The method of claim 1 wherein detecting tachyarrhythmia using signalsfiltered by the first filter in combination with signals filtered by thesecond filter includes: comparing ventricular channel signals filteredby the first filter with ventricular channel signals filtered by thesecond filter to distinguish between true ventricular depolarizationevents and false ventricular depolarization events; and detectingtachyarrhythmia based on the true ventricular depolarization events. 7.The method of claim 1 wherein the device additionally includes awideband filter having a substantially wider bandwidth than bandwidthsof the first and second filters and wherein the step of detectingtachyarrhythmia uses signals filtered by the wideband filter incombination with signals filters by the first and second filters.
 8. Themethod of claim 1 further including delivering therapy in response tothe tachyarrhythmia.
 9. The method of claim 1 wherein the first filteris a bradycardia filter and wherein the second filter is a tachycardiafilter and wherein detecting tachyarrhythmia within the patient isperformed using signals filtered by the bradycardia filter incombination with signals filtered by the tachycardia filter.
 10. Themethod of claim 1 wherein the first filter has a passband beginning atabout 20 Hz and wherein the second filter has a passband beginning atabout 10 Hz and wherein detecting tachyarrhythmia within the patient isperformed using signals filtered by the first filter having the passbandbeginning at about 20 Hz in combination with signals filtered by thesecond filter having the passband beginning at about 10 Hz.
 11. Atachyarrhythmia detection system for use in an implantable medicaldevice having filters for filtering electrical cardiac signals sensedvia leads implanted within the heart of a patient, the systemcomprising: a first filter operative to substantially eliminate signalshaving frequencies associated with ventricular repolarization eventswhile retaining signals having frequencies associated with at least someventricular depolarization events; a second filter operative to passsignals having frequencies associated with ventricular depolarizationevents and ventricular repolarization events; and a combined firstfilter/second filter-based arrhythmia detection system operative todetect tachyarrhythmia within the patient using signals filtered by thefirst filter in combination with the signals filtered by the secondfilter.
 12. The system of claim 11 wherein the combined firstfilter/second filter-based detection system includes: a firstfilter-based preliminary detection unit operative to detect apreliminary indication of tachyarrhythmia using signals filtered by thefirst filter; and a second filter-based confirmation unit to thenconfirm the detection of tachyarrhythmia using signals filtered by thesecond filter.
 13. The system of claim 11 wherein the combined firstfilter/second filter-based detection system includes a comparison unitoperative to compare the ventricular channel signals filtered by thesecond filter and the first filter to detect ventriculartachyarrhythmia.
 14. The system of claim 11 wherein the combined firstfilter/second filter-based detection system includes: a secondfilter-based preliminary detection unit operative to detect apreliminary indication of tachyarrhythmia using signals filtered by thesecond filter; and a duel filter-based confirmation unit to then confirmthe detection of tachyarrhythmia using signals filtered by the firstfilter and the second filter.
 15. The system of claim 11 wherein thecombined first filter/second filter-based detection system includes: asecond filter-based preliminary detection unit operative to detect apreliminary indication of tachyarrhythmia using signals filtered by thefirst filter; and a second filter-based confirmation unit operative todetect oversensing of ventricular repolarization events by the secondfilter and to confirm the detection of tachyarrhythmia only in theabsence of oversensing of ventricular repolarization events.
 16. Thesystem of claim 11 wherein the implantable device additionally includesa wideband filter having a substantially wider bandwidth than bandwidthsof the first and second filters; and wherein the combined firstfilter/second filter-based detection system includes a comparison unitoperative to detect tachyarrhythmia using signals filtered by thewideband filter in combination with signals filters by the first andsecond filters.
 17. A tachyarrhythmia detection system for use in animplantable medical device having filters for filtering electricalcardiac signals sensed via leads implanted within the heart of apatient, the system comprising: means for sensing electrical cardiacsignals within the patient; first means for filtering the signals tosubstantially eliminate signals having frequencies associated withventricular repolarization events while retaining signals havingfrequencies associated with at least some ventricular depolarizationevents; second means for filtering the signals to pass signals havingfrequencies associated with ventricular depolarization events andventricular repolarization events; and means for detectingtachyarrhythmia within the patient by comparing the first means forfiltering and the second means for filtering.